remove man3x from from xorg-macros.m4

man3x dir does not exist, and if man pages refer to man3x via man links,
the manual pages can't be retrieved

see:
http://git.pld-linux.org/?p=packages/xorg-lib-libXt.git;a=commitdiff;h=56aa5d5d5b17dd3b891ef00678eff52c9490ec1d