pid: cleanup the stale comment mentioning pidmap_init().

pidmap_init() has already been replaced with pid_idr_init() in the commit
95846ecf

 ("pid: replace pid bitmap implementation with IDR API").
Cleanup the stale comment which still mentions it.

Link: https://lkml.kernel.org/r/20210714120713.19825-1-itazur@amazon.com
Signed-off-by: Takahiro Itazuri <itazur@amazon.com>
Cc: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>