oprofile: remove undocumented oprofile.p4force option

There are no new P4s and the oprofile code knows about all existing
ones, so we don't really need the p4force option anymore.

Remove it.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>