Documentation: kselftests: Remove references to bpf tests

Currently the bpf selftests are skipped by default, so is someone would
like to run the tests one would need to run:
  $ make TARGETS=bpf SKIP_TARGETS="" kselftest

To overwrite the SKIP_TARGETS that defines bpf by default. Also,
following the BPF instructions[1], to run the bpf selftests one would
need to enter in the tools/testing/selftests/bpf/ directory, and then
run make, which is not the standard way to run selftests per it's
documentation.

For the reasons above stop mentioning bpf in the kselftests as examples
of how to run a test suite.

[1]: Documentation/bpf/bpf_devel_QA.rst

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>