contrib: remove git-p4import

The git p4import documentation has suggested git p4 as a better
alternative for more than 6 years.  (According to the mailing list
discussion when it was moved to contrib/, git-p4import has serious
bugs --- e.g., its incremental mode just doesn't work.) Since then,
git p4 has been actively developed and was promoted to a standard git
command alongside git svn.

Searches on google.com/trends and stackoverflow suggest that no one is
looking for git-p4import any more.  Remove it.

Noticed while considering marking the contrib/p4import/git-p4import.py
script executable as part of a wider sweep.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>