char-pty: remove the check for connection on write

This doesn't help much compared to the 1 second poll PTY
timer. I can't think of a use case where this would help.

However, we can simplify the code around chr_write(): the write lock
is no longer needed for other char-pty callbacks (see following
patch).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190206174328.9736-6-marcandre.lureau@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>