remove dead portions of BpfUtils.java

(mostly to improve code coverage)

We can always revert if it ever turns out this is useful,
but I'd prefer for cgroup attach to be a once-at-boot
thing and handled either from bpfloader or the netd updatable
which are both directly C++ code.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I79b5ca8a47388cb6b9189234942e74ab6056aab9