selftests/bpf: DENYLIST.aarch64: Remove fexit_sleep

fexit_sleep test runs successfully now on the BPF CI so remove it
from the deny list. ftrace direct calls was blocking tracing programs
on arm64 but it has been resolved by now. For more details see also
discussion in [*].

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240705145009.32340-1-puranjay@kernel.org [*]