block: Don't disable I/O throttling on sync requests

We had to disable I/O throttling with synchronous requests because we
didn't use to run timers in nested event loops when the code was
introduced. This isn't true any more, and throttling works just fine
even when using the synchronous API.

The removed code is in fact dead code since commit a8823a3b ('block: Use
blk_co_pwritev() for blk_write()') because I/O throttling can only be
set on the top layer, but BlockBackend always uses the coroutine
interface now instead of using the sync API emulation in block.c.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <1458660792-3035-2-git-send-email-kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
