Revert "seccomp: add a selftest for get_metadata"

This reverts commit e65cd9a2.

Tommi T. Rrantala notes:

	PTRACE_SECCOMP_GET_METADATA was only added in 4.16
	(26500475ac1b499d8636ff281311d633909f5d20)

	And it's also breaking seccomp_bpf.c compilation for me:

	seccomp_bpf.c: In function ‘get_metadata’:
	seccomp_bpf.c:2878:26: error: storage size of ‘md’ isn’t known
	  struct seccomp_metadata md;

Signed-off-by: Sasha Levin <sashal@kernel.org>