bug#3973: cleanup view-mode usage in vc-resynch-window

Stefan Monnier <monnier@xxxxxxxxxxxxxxxx> writes:

  > > I have removed that code locally for a while and didn't see any
  > > problem.  But I don't explicitly use view-mode, so...
  > 
  > Obiously the code only makes a difference when you use view-read-only,
  > so you'll need to set it to a non-nil value before doing any test.

[I've never done anything with view-read-only before...]
I set view-read-only to t, then played with a file under RCS.
When the files is checked in view-mode is turned on, when checking it
out it's turned off.

If the code in question is eliminated, view-mode stays turned on after
checking out.

There's some code in `after-find-file' (that gets called from
`revert-buffer') that looks like it does something very similar to the
code in question here, but `after-find-file' is called with `nomodes' t,
so that code does not trigger.