xserver_xorg-server: Fix race condition when installing man pages, 2nd try

Should fix the still present bug:
http://autobuild.buildroot.net/results/045/04514bc9a8d1aad8ddd7af4f0fd8c16edd2e89a9/
http://autobuild.buildroot.net/results/1f2/1f25f744771fbb5124dca4563d79ea92cbfd72a3/

This patch reverts my previous attempt to fix this hard-to-catch race condition
http://git.buildroot.net/buildroot/commit/package/x11r7/xserver_xorg-server?id=e244eb12dfc50bb56167d5fdaef3aac52b0ff6a3

and removes make target install-data from _INSTALL_STAGING_OPT, which was
introduced in 2008:
http://git.buildroot.net/buildroot/commit/package/x11r7/xserver_xorg-server/xserver_xorg-server.mk?id=d4e56ff4dc8a0528550eb8503956bdcddb2fe516



Comparing the content of STAGING_DIR with and without "make install-data" showed
no difference so I assume that is it no longer necessary to explicitly call this
make target as the normal install target takes care of everything.

The race condition occurs because "make install" and "make install-data" both
install usr/share/man/man1/Xserver.1

[Peter: don't remove host-xfont_font-util]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>