Remove "Tag: " prefix, now that they're in MW core

When If21eaed41 gets merged, the custom of writing "Tag: Foo" will be obviated
by MediaWiki core having this itself. Consequently, once that is merged, we should
remove them from VisualEditor's messages. (Not removing from the non-English
messages per standing guidance from the Language Engineering team.)

Bug: 49602
Change-Id: Ibdd8053c1df950522379f8b05d260b71593d4741
Merge "Remove "Tag: " prefix, now that they're in MW core"