rm: remove unneeded null pointer check

rm: remove unneeded null pointer check

As of 7612a1efdb (2006-06-09 git-rm: honor -n flag.) the variable
'pathspec' seems to be assumed to be never NULL after calling get_pathspec
There was a NULL pointer check after the seen = NULL assignment, which
was removed by that commit. So if pathspec would be NULL now, we'd segfault
in the line accessing the pathspec:
	for (i = 0; pathspec[i] ; i++)

A few lines later, 'pathspec' still cannot be NULL, but that check was
overlooked, hence removing it now.

As the null pointer check was removed, it makes no sense to assign NULL
to seen and 3 lines later another value as there are no conditions in
between.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>