manifestdict.walk: remove now-redundant check for match.files()
When checking whether we can take the fast path of iterating over
matcher files instead of manifest files, we check whether
match.files() is non-empty. However, now that return early for
match.always(), it can only be empty when there are only
include/exclude patterns, but in that case anypats() will be True, so
it's already covered. This makes manifestdict.walk() more similar to
manifestdict.matches().
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown