network-manager: remove bogus --mandir option

network-manager: remove bogus --mandir option

The network-manager.mk passes --mandir=$(STAGING_DIR)/usr/man/, which
causes man pages to be installed in
$(STAGING_DIR)/$(STAGING_DIR)/usr/man and
$(TARGET_DIR)/$(STAGING_DIR)/usr/man.

This is definitely bogus and causes some build failures thanks to the
new check added by Yann E. Morin. This option was in
network-manager.mk since the addition of the package. Maybe it was
fixing a bug in the package logic back at the time, but nowadays, it
causes an installation at the wrong location. Without this commit, the
man pages are properly installed at the right location.

Fixes:

  http://autobuild.buildroot.net/results/92683749b0b8e726bc83781ca6d8e103428362ba/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>