selftests/bpf: Don't mask result of bpf_csum_diff() in test_verifier

The bpf_csum_diff() helper has been fixed to return a 16-bit value for
all archs, so now we don't need to mask the result.

This commit is basically reverting the below:

commit 6185266c5a ("selftests/bpf: Mask bpf_csum_diff() return value
to 16 bits in test_verifier")

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20241026125339.26459-4-puranjay@kernel.org

tools/testing/selftests/bpf/progs/verifier_array_access.c

@@ -368,8 +368,7 @@ __naked void a_read_only_array_2_1(void)
 	r4 = 0;						\
 	r5 = 0;						\
 	call %[bpf_csum_diff];				\
-l0_%=:	r0 &= 0xffff;					\
-	exit;						\
+l0_%=:	exit;						\
 "	:
 	: __imm(bpf_csum_diff),
 	  __imm(bpf_map_lookup_elem),