config.mak.in: remove unused definitions

config.mak.in: remove unused definitions

When 5566771 (autoconf: Use autoconf to write installation
directories to config.mak.autogen, 2006-07-03) introduced support
for autoconf generated config.mak file, it added an "export" for a
few common makefile variables, in addition to definitions of srcdir
and VPATH.

The "export" logically does not belong there.  The make variables
like mandir, prefix, etc, should be exported to submakes for people
who use config.mak and people who use config.mak.autogen the same
way; if we want to get these exported, that should be in the main
Makefile.

We do use mandir and htmldir in Documentation/Makefile, so let's
add export for them in the main Makefile instead.

We may eventually want to support VPATH, and srcdir may turn out to
be useful for that purpose, but right now nobody uses it, so it is
useless to define them in this file.

Signed-off-by: Junio C Hamano <gitster@pobox.com>