This reverts commit d34f4e83.

This does not work as desired because all current I-pipe versions have
the IPIPE_KEVT_USERINTRET define while some (namely arm64 and ppc32 ATM)
do not implement the callback. The latter is a bug and needs fixing, but
not here.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>