trim --prune: Look at FLAG_NO_SIDE_EFFECTS rather than FLAG_VERBOSE

The --prune option is documented precisely to prune calls with no side
effects, so that's the flag it should examine.

Previously, the trim --prune code was keying off of the VERBOSE
flag. This hasn't made a substantial difference in the past, (the
NO_SIDE_EFFECTS calls have historically all been VERBOSE as well). But
NO_SIDE_EFFECTS has the advantage that it can objectively determined,
so the set of calls that should have this flag should be
unambiguous. (Meanwhile, whether any particular call should be
classified as VERBOSE is inherently subjective.)