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 <redacted> Signed-off-by: David Aguilar <redacted> Signed-off-by: Junio C Hamano <redacted>