Delete reindex exception change todos (#49271)

This is related to #42612. This commit deletes TODOs related to test
assertions changing due to exception serialization changes. We have
determined that the exception x-content serialization was never stable
in the first place (reading from the task index), so we are okay with
these changes.

modules/reindex/src/test/resources/rest-api-spec/test/reindex/10_basic.yml

@@ -158,7 +158,6 @@
   - match: {failures.0.index:  dest}
   - match: {failures.0.id:     "1"}
   - match: {failures.0.status: 409}
-  # TODO: Changed the message because of exception serialization
   - match: {failures.0.cause.type:   exception}
   # Use a regex so we don't mind if the version isn't always 1. Sometimes it comes out 2.
   - match: {failures.0.cause.reason: "/\\[1\\]:.version.conflict,.document.already.exists.\\(current.version.\\[\\d+\\]\\)/"}

modules/reindex/src/test/resources/rest-api-spec/test/reindex/35_search_failures.yml

@@ -28,11 +28,8 @@
                   source: throw new IllegalArgumentException("Cats!")
           dest:
             index:   dest
-  # TODO: revisit this once we keep original type.
   - match: {error.root_cause.0.type: exception}
-  # TODO: revisit this once we keep original reason message.
   - match: {error.root_cause.0.reason: "Elasticsearch exception [type=search_phase_execution_exception, reason=Partial shards failure]"}
   - match: {error.root_cause.0.phase: query}
   - match: {error.type: status_exception}
   - match: {error.reason: "Elasticsearch exception [type=search_phase_execution_exception, reason=Partial shards failure]"}
-  # TODO: we kind of change the response format here, but we never documented this so OK?

modules/reindex/src/test/resources/rest-api-spec/test/reindex/85_scripting.yml

@@ -439,5 +439,4 @@
           script:
             lang: painless
             source: syntax errors are fun!
-  # TODO: Changed the message because of exception serialization
   - match: {error.reason: 'Elasticsearch exception [type=script_exception, reason=compile error]'}