bpf: Remove runqslower tool

bpf: Remove runqslower tool

runqslower was added in commit 9c01546d26d2 "tools/bpf: Add runqslower
tool to tools/bpf" as a BCC port to showcase early BPF CO-RE + libbpf
workflows. runqslower continues to live in BCC (libbpf-tools), so there
is no need to keep building and maintaining it.

Drop tools/bpf/runqslower and remove all build hooks in tools/bpf and
selftests accordingly.

Signed-off-by: Hoyeon Lee <hoyeon.lee@suse.com>
Link: https://lore.kernel.org/r/20251126093821.373291-1-hoyeon.lee@suse.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>