libc: remove useless code in _prf()

Coverity reported a block of deadcode in _prf() that seems to be a
leftover carcass from a previous time. Replaced with a comment in case
someone decides it was needed back.

Change-Id: Id97e84f3279f807e6188371f27f6af157e6d5038
Coverity-ID: 131631
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>