bpf, selftests: test_maps generating unrecognized data section

With a relatively recent clang master branch test_map skips a section,

 libbpf: elf: skipping unrecognized data section(5) .rodata.str1.1

the cause is some pointless strings from bpf_printks in the BPF program
loaded during testing. After just removing the prints to fix above error
Daniel points out the program is a bit pointless and could be simply the
empty program returning SK_PASS.

Here we do just that and return simply SK_PASS. This program is used with
test_maps selftests to test insert/remove of a program into the sockmap
and sockhash maps. Its not testing actual functionality of the TCP
sockmap programs, these are tested from test_sockmap. So we shouldn't
lose in test coverage and fix above warnings. This original test was
added before test_sockmap existed and has been copied around ever since,
clean it up now.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/161731595664.74613.1603087410166945302.stgit@john-XPS-13-9370