fsck docs: remove outdated and useless diagnostic

fsck docs: remove outdated and useless diagnostic

In git-fsck(1), there was a reference to the warning "<tree> has full
pathnames in it".  This exact wording has not been used since 2005
(commit f1f0d0889e55), when the wording was changed slightly.  More
importantly, the description of that warning was useless, and there were
many other similar warning messages which were not document at all.
Since all these warnings are fairly obvious, there is no need for them
to be in the man page.

Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>