libprocessgroup: Remove prctl interface for setting timer slack

Support for the procfs timerslack interface has existed since linux 4.6.
All currently supported Android kernels are newer than 4.6, so remove
the userspace support for the prctl interface leaving just the use of
/proc/<tid>/timerslack_ns.

Bug: 372498744
Change-Id: Ib34cc6fbb4bab328cde51d60ac4e3b26a0d72851