git-mergetool: remove redundant assignment

TOOL_MODE is set at the top of git-mergetool.sh so there is no need to
set it again in show_tool_help.  Removing this lets us re-use
show_tool_help in git-difftool.

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

git-mergetool--lib.sh

@@ -175,7 +175,6 @@ list_merge_tool_candidates () {
 }
 
 show_tool_help () {
-	TOOL_MODE=merge
 	list_merge_tool_candidates
 	unavailable= available= LF='
 '