perf: Clean up redundant group validation

None of these drivers are doing anything that perf_event_open() doesn't
inherently do as of commit bf480f93 ("perf/core: Don't allow
grouping events from different hw pmus"). While it's quite possible
that they should be doing some actual validation of the schedulability
of their own events within the given group, for now at least removing
this redundant code makes it even clearer that they are not.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>