compat: remove compat_printk()

After 7e8e385a ("x86/compat: Remove sys32_vm86_warning"), this
function has become unused, so we can remove it as well.

Link: http://lkml.kernel.org/r/20160617142903.3070388-1-arnd@arndb.de


Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>