Makefile: do not replace @@GIT_USER_AGENT@@ in scripts

No scripts actually care about this replacement. This was
erroneously added by 42dcbb73 (version: add git_user_agent function,
2012-06-02).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -2007,7 +2007,6 @@ sed -e '1s|#!.*/sh|#!$(SHELL_PATH_SQ)|' \
     -e 's|@SHELL_PATH@|$(SHELL_PATH_SQ)|' \
     -e 's|@@DIFF@@|$(DIFF_SQ)|' \
     -e 's/@@GIT_VERSION@@/$(GIT_VERSION)/g' \
-    -e 's|@@GIT_USER_AGENT@@|$(GIT_USER_AGENT_SQ)|g' \
     -e 's|@@LOCALEDIR@@|$(localedir_SQ)|g' \
     -e 's/@@NO_CURL@@/$(NO_CURL)/g' \
     -e 's/@@USE_GETTEXT_SCHEME@@/$(USE_GETTEXT_SCHEME)/g' \