samples/bpf: remove obsolete cgroup related tests

samples/bpf: remove obsolete cgroup related tests

This patch removes the obsolete cgroup related tests. These tests are
now redundant because their functionality is already covered by more
modern and comprehensive tests under selftests/bpf.

The following files are removed:
- test_current_task_under_cgroup: tests bpf_current_task_under_cgroup()
  to check if a task belongs to a cgroup. Already covered by
  task_under_cgroup at selftest and other cgroup ID tests.

- test_cgrp2_tc: tests bpf_skb_under_cgroup() to filter packets based
  on cgroup. This behavior is now validated by cgroup_skb_sk_lookup,
  which uses bpf_skb_cgroup_id, making this test redundant.

By removing these outdated tests, this patch helps streamline and
modernize the test suite, avoiding duplication of test coverage.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Link: https://lore.kernel.org/r/20241011044847.51584-4-danieltimlee@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>