libprocessgroup: Remove ifdefery around SetTimerSlackAction::ExecuteForTask

This appears to have been due to the use of prctl, which apparently
caused issues for mac builds. Now that use of prctl has been removed, it
doesn't look like we need this anymore.

Fixes: 07500817 ("libprocessgroup: Remove prctl interface for setting timer slack")
Test: m
Bug: 372498744
Change-Id: I8a43951f30d7dd838591a8ba225d712742d7cd4a