[fix](Nereids) remove db readlock before get table from db (apache#38660

) (apache#38729)

pick from master apache#38660

insert will hold readlock of target table before planning. if nereids
need db readlock after it, will lead to dead lock. because other
statement need to hold db lock before get table lock

for example:

insert: target table read lock -> database read lock
drop table: database write lock -> target table write lock

fe/fe-core/src/main/java/org/apache/doris/nereids/CascadesContext.java

@@ -552,16 +552,13 @@ public TableIf getTable(String ctlName, String dbName, String tableName, Env env
         if (db == null) {
             throw new RuntimeException("Database [" + dbName + "] does not exist in catalog [" + ctlName + "].");
         }
-        db.readLock();
-        try {
-            TableIf table = db.getTableNullable(tableName);
-            if (table == null) {
-                throw new RuntimeException("Table [" + tableName + "] does not exist in database [" + dbName + "].");
-            }
-            return table;
-        } finally {
-            db.readUnlock();
+
+        TableIf table = db.getTableNullable(tableName);
+        if (table == null) {
+            throw new RuntimeException("Table [" + tableName + "] does not exist in database [" + dbName + "].");
         }
+        return table;
+
     }
 
     /**