watchman: drop some py2 compat code
The `unicode` reference was being flagged by pytype, even though it was never
evaluated on py3. There's more that can be dropped and `compat.py` can probably
be inlined if we don't care about minimizing the code changes from FB. But I
don't feel like dealing with that.
https://bz.mercurial-scm.org/1089
$ hg init repo
$ cd repo
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..