Log: libcrmcommon: Drop crm_perror() in procfs.c

The controller is the only caller of pcmk__procfs_num_cores(). We don't
typically see stderr from daemons. So we're not really losing anything
by not writing this message to stderr as crm_perror() did.

Ref T172

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>