autofs: remove AUTOFS_DEVID_LEN

This macro was never used by neither kernel nor userspace, and also
doesn't represent "devid length" in bytes.  (unless it was added to mean
something else).

Link: http://lkml.kernel.org/r/20160812024820.12352.21210.stgit@pluto.themaw.net


Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Ian Kent <ikent@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>