autofs: delete fs/autofs4 source files

Delete the now unused autofs4 module files.

Link: http://lkml.kernel.org/r/152626707391.28589.3553309771262313504.stgit@pluto.themaw.net
Signed-off-by: Ian Kent <raven@themaw.net>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>