trim: Don't consider a SwapBuffers call has having no side effects

I can't justify why I ever added this code. I can only guess that I was
trying to get qapitrace to not show frame markers for frames that had
largely been trimmed.

Anyway, the code is wrong. SwapBuffers call obviously have significant
side effects. And the effect I was trying to achieve is already well
satisified by the change in 2f0d1a3244c8953a4468759ac466b80c4965d38f
that will trim a SwapBuffers call when --trim-spec=drawing is in
force.