dirstate: drop unused condition in `from_p2`
This conditional was added (by me) tentatively because "it seemed more
correct", but it is not used anywhere yet, and it is missing from the C and the
Rust implementation. So it seems more consistent to drop it for now.
This effectively backout f94cc63df859c
Differential Revision: https://phab.mercurial-scm.org/D11510
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls -A
.hg
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls -A
.hg
a