[front end] Remove message... declarations.

This is the final CL in a series of CLs that standardizes CFE error
reporting to always use `codeFoo.withArguments(...)` when reporting
errors that take arguments and `codeFoo` when reporting errors that
don't take arguments, rather than `templateFoo.withArguments(...)`
when reporting errors that take arguments and `messageFoo` when
reporting errors that don't take arguments. This change has two
advantages:

- It lends greater consistency to the CFE codebase, by allowing the
  same `code...` objects to be used both to name error codes (e.g., in
  test expectations) and to report errors. This allows everything
  associated with a certain error code to be found using a single
  invocation of "Find References" in the editor, rather than having to
  search separately for uses of the code and the message or template.

- It should hopefully make the experience of writing code that reports
  errors more pleasant, since it will no longer be necessary to look
  up an error to see whether it takes arguments before using it;
  instead, the developer will be able to type the name of the message
  `code...` declaration, and then use autocompletion to see whether
  `.withArguments(...)` is required.

In this CL, the `message...` declarations that define errors are
removed, since they are no longer used (the equivalent `code...`
declarations are used instead).

Change-Id: I404e240d8d376556f147a7d80c6953bca5222f61
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/443180
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441