git_remote_helpers: remove GIT-PYTHON-VERSION upon "clean"

fadf8c7 (git_remote_helpers: force rebuild if python version changes, 2013-01-20)
started using a marker file to keep track of the version of Python interpreter
used for the last build, but forgot to remove it when asked to "make clean".

Acked-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>