Improve performance of delete-selection-pre-hook when no region is active

[rgemulla]

This is a small change to the delete-selection-pre-hook advice to avoid unnecessary computations.

I noticed sluggish responses when moving around some buffers; profiling revealed the advice to delete-selection-pre-hook to be the culprit. In particular, it repeatedly called sp-wrap--can-wrap-p, which ate a large fraction of CPU time. With this patch, sp-wrap--can-wrap-p is only called when there actually is an active region. The additional condition is directly taken from delete-selection-pre-hook; it is used there to decide whether to do something. With this small change, the problem of sluggish response times went away.

[Fuco1]

Thanks, nice catch!