scripts/setlocalversion: clean up stale comment

Nobody has complained since 2a73cce2 ("scripts/setlocalversion:
remove mercurial, svn and git-svn supports"), so let's also clean up
the header comment accordingly.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>