Remove support for namespace attribute (#6279)

## Description
Namespace annotation is no longer supported and has been replaced by
namespacing syntax.


## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [x] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

examples/storage_namespace/src/main.sw

@@ -3,10 +3,11 @@ contract;
 use std::storage::storage_api::{read, write};
 
 // ANCHOR: storage_namespace
-#[namespace(example_namespace)]
 storage {
+    example_namespace {
+        foo: u64 = 0,
+    },
     // ANCHOR_END: storage_namespace
-    foo: u64 = 0,
 }
 
 abi StorageNamespaceExample {

sway-core/src/transform/to_parsed_lang/convert_parse_tree.rs

@@ -34,9 +34,9 @@ use sway_types::{
         ALLOW_ATTRIBUTE_NAME, CFG_ATTRIBUTE_NAME, CFG_EXPERIMENTAL_NEW_ENCODING,
         CFG_PROGRAM_TYPE_ARG_NAME, CFG_TARGET_ARG_NAME, DEPRECATED_ATTRIBUTE_NAME,
         DOC_ATTRIBUTE_NAME, DOC_COMMENT_ATTRIBUTE_NAME, FALLBACK_ATTRIBUTE_NAME,
-        INLINE_ATTRIBUTE_NAME, NAMESPACE_ATTRIBUTE_NAME, PAYABLE_ATTRIBUTE_NAME,
-        STORAGE_PURITY_ATTRIBUTE_NAME, STORAGE_PURITY_READ_NAME, STORAGE_PURITY_WRITE_NAME,
-        TEST_ATTRIBUTE_NAME, VALID_ATTRIBUTE_NAMES,
+        INLINE_ATTRIBUTE_NAME, PAYABLE_ATTRIBUTE_NAME, STORAGE_PURITY_ATTRIBUTE_NAME,
+        STORAGE_PURITY_READ_NAME, STORAGE_PURITY_WRITE_NAME, TEST_ATTRIBUTE_NAME,
+        VALID_ATTRIBUTE_NAMES,
     },
     integer_bits::IntegerBits,
     BaseIdent,
@@ -4698,13 +4698,6 @@ fn item_attrs_to_map(
         let attrs = attr_decl.attribute.get().into_iter();
         for attr in attrs {
             let name = attr.name.as_str();
-            if name == NAMESPACE_ATTRIBUTE_NAME {
-                handler.emit_warn(CompileWarning {
-                    span: attr_decl.span().clone(),
-                    warning_content: Warning::NamespaceAttributeDeprecated,
-                });
-                continue;
-            }
             if !VALID_ATTRIBUTE_NAMES.contains(&name) {
                 handler.emit_warn(CompileWarning {
                     span: attr_decl.span().clone(),

sway-types/src/constants.rs

@@ -53,8 +53,6 @@ pub const CFG_EXPERIMENTAL_NEW_ENCODING: &str = "experimental_new_encoding";
 
 pub const DEPRECATED_ATTRIBUTE_NAME: &str = "deprecated";
 
-pub const NAMESPACE_ATTRIBUTE_NAME: &str = "namespace";
-
 pub const FALLBACK_ATTRIBUTE_NAME: &str = "fallback";
 
 /// The list of valid attributes.

test/src/sdk-harness/test_projects/storage_namespace/src/main.sw

@@ -47,7 +47,6 @@ const S_13: b256 = 0x00000000000000000000000000000000000000000000000000000000000
 const S_14: b256 = 0x0000000000000000000000000000000000000000000000000000000000000014;
 const S_15: b256 = 0x0000000000000000000000000000000000000000000000000000000000000015;
 
-#[namespace(my_storage_namespace)]
 abi StorageTest {
     #[storage(read, write)]
     fn store_bool(value: bool);