SF: Remove forced transaction on screenshot

Removes the forced transaction that screenshots used to perform
before capturing the screen. This was initially added to prevent
screenshot capture from sneaking in between when a transaction is
closed and when it is actually applied (at the next vsync), but in
the presence of deferred transactions today, forcing a transaction
to occur doesn't guarantee that the last transaction that
WindowManager pushed has actually been committed yet, since it may be
waiting for a buffer with which to synchronize.

If WindowManager wants to guarantee some arrangement of windows is
present for a screenshot, it needs to set the synchronous flag to
true when calling closeGlobalTransaction and wait for the call to
return.

Bug: 27098060
Change-Id: Iaa36d76df521076cc8ed16576f651543bae419a8