gitignore: remove entry for git serve

b7ce24d095 (Turn `git serve` into a test helper, 2019-04-18) demoted git
serve from a builtin command to a test helper.  As a result the
git-serve binary is no longer built and thus doesn't have to be ignored
anymore.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitignore

@@ -147,7 +147,6 @@
 /git-rm
 /git-send-email
 /git-send-pack
-/git-serve
 /git-sh-i18n
 /git-sh-i18n--envsubst
 /git-sh-setup