selftests/bpf: Drop an unused local variable

Some copy/paste leftover, this is never used.

Fixes: e3d9eac9 ("selftests/bpf: wq: add bpf_wq_init() checks")
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/bpf/20240430-bpf-next-v3-3-27afe7f3b17c@kernel.org