new_library: remove unused resolver (#3219)

`_testmain_library_to_source` was introduced in 1c4f6fd1 and last usage
of this resolver was in 56e5592c.

  ```
  > git log -S'_testmain_library_to_source' --oneline
  56e5592c Move the test library rule to be go_test internal actions (#1267)
  9031d58e Add GoContext (#1140)
  1c4f6fd1 Fix aspect based proto builds (#1131)
  ```

Let's remove the unused resolver and replace the documentation with the
resolver that is actually being used.
2 files changed