samples/bpf: Drop unnecessary fallthrough

__fallthrough is now not supported. Instead of renaming it to
now-canonical ([0]) fallthrough pseudo-keyword, just get rid of it and
equate 'h' case to default case, as both emit usage information and
succeed.

  [0] https://www.kernel.org/doc/html/latest/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through



Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20230516001718.317177-1-andrii@kernel.org