Makefile: drop 'rebuild' from help message

The target has been broken for some time without complaints from users.
https://lists.openvz.org/pipermail/criu/2016-March/026525.html

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

Makefile

@@ -235,7 +235,6 @@ help:
 	@echo '      tags            - Generate tags file (ctags)'
 	@echo '      etags           - Generate TAGS file (etags)'
 	@echo '      cscope          - Generate cscope database'
-	@echo '      rebuild         - Force-rebuild of [*] targets'
 	@echo '      test            - Run zdtm test-suite'
 	@echo '      gcov            - Make code coverage report'
 .PHONY: help