docs: Remove README-hg.md

It's been a few years in general, and SDL3 has never been in Mercurial.

We'll leave this document alive in the SDL2 branch, though!

docs/README-hg.md

@@ -1,7 +0,0 @@
-Mercurial
-======
-
-We are no longer hosted in Mercurial. Please see README-git.md for details.
-
-Thanks!
-