tests: fix test-update-branches to remove non-conflicting file

I was clearly very sloppy when I wrote the test case for
experimental.updatecheck=noconflict. The test case that checks that
one can move between commits with a removed file was deleting a file
that was modified between the source and target commits, which
resulted in a "change/delete" conflict. Since that is a conlict, the
update correctly failed. Let's fix the test by removing a file that is
not modified between the two commits.

tests/test-update-branches.t

@@ -288,11 +288,8 b' Locally added file is allowed'
 
 Locally removed file is allowed
   $ hg up -qC 3
-  $ hg rm a
+  $ hg rm foo
   $ hg up -q 4
-  abort: uncommitted changes
-  (commit or update --merge to allow merge)
-  [255]
 
 File conflict is not allowed
   $ hg up -qC 3