[messages] Remove utility to convert to camelCase.

Now that diagnostic codes in `messages.yaml` files are required to be
`camelCase`, this utility isn't needed anymore.

Change-Id: I6a6a69646f93abf677abd46d53db4c17945c2c6f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/467100
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/analyzer_utilities/pubspec.yaml

@@ -21,6 +21,5 @@ dependencies:
 
 # Use 'any' constraints here; we get our versions from the DEPS file.
 dev_dependencies:
-  analyzer_plugin: any
   lints: any
   test_reflective_loader: any