git: remove obsolete todo item

The changes in 02c47b74366c cleaned up the requirement check.

hgext/git/TODO.md

@@ -28,12 +28,3 We should spend some time thinking hard
 repository, but may not have enough locking correctness in places
 where hg does locking that git isn't aware of (notably the working
 copy, which I believe Git does not lock.)
-
-Clean up requirements
-=====================
-
-Right now (for historical reasons, mainly) hgext.git uses a
-.hg/this-is-git file to detect repositories that should be treated as
-git. We should look in the .hg/requires for the "git" requirement
-instead (we already set this requirement, so it's mostly keying off
-that instead of using an empty file.)