Remove unused variables from fs/proc/base.c

When removing the explicit task_struct->pid usage I found that
proc_readfd_common() and proc_pident_readdir() get this field, but do not
use it at all.  So this cleanup is a cheap help with the task_struct->pid
isolation.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>