meson: remove obsolete check for LOOP_CTL_GET_FREE

The LOOP_CTL_GET_FREE constant was introduced to Linux in

  commit 770fe30a46a12b6fb6b63fbe1737654d28e84844
  Author: Kay Sievers <kay.sievers@vrfy.org>
  Date:   Sun Jul 31 22:08:04 2011 +0200

    loop: add management interface for on-demand device allocation

This is old enough that all our supported platforms can be assumed
to have this feature. As a plus point, this meson check is going
to start failing with future GCC. It fails to set _GNU_SOURCE, thus
'unshare' is not defined by the header, and its relying on an
implicit function decl. For added fun this whole meson check was
semantically insane because LOOP_CTL_GET_FREE is not a valid arg
to unshare().

Fixes https://fedoraproject.org/wiki/Toolchain/PortingToModernC
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

meson.build

@@ -1541,19 +1541,6 @@ void main(void) {
   elif get_option('driver_lxc').enabled()
     error('Required kernel features for LXC were not found')
   endif
-
-  lxc_get_free_code = '''
-#include <sched.h>
-#include <linux/loop.h>
-#include <sys/epoll.h>
-
-void main(void) {
-    unshare(!(LOOP_CTL_GET_FREE));
-}
-  '''
-  if cc.compiles(lxc_get_free_code)
-    conf.set('WITH_DECL_LOOP_CTL_GET_FREE', 1)
-  endif
 elif get_option('driver_lxc').enabled()
   error('linux and remote_driver are required for LXC')
 endif