git.eclass: drop old/unused eclass

Everything in the tree uses newer git-2/git-r3, so drop this.

BUG=chromium:1042641
TEST=CQ passes

Change-Id: I3a196ee0f2d8ded86d408d1bf7747feab6f07464
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2195741
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>