git-gui: remove uname_O in Makefile

Last used in ae49066 (git gui Makefile - remove Cygwin modifications,
2023-06-26), and unused since.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>

Author: carenas

Makefile

@@ -11,7 +11,6 @@ GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN . $@
 
 uname_S := $(shell sh -c 'uname -s 2>/dev/null || echo not')
-uname_O := $(shell sh -c 'uname -o 2>/dev/null || echo not')
 uname_R := $(shell sh -c 'uname -r 2>/dev/null || echo not')
 
 SCRIPT_SH = git-gui.sh