selftests/bpf: remove test_flow_dissector.sh

Now that test_flow_dissector.sh has been converted to test_progs, remove
the legacy test.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20241120-flow_dissector-v3-14-45b46494f937@bootlin.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>