TrafficController - trivialize dump()

The goal is to be able to remove getMap() from BpfMap.h,
as it effectively leaks (what should be an) internal fd.

In practice the dump() operation isn't useful,
as initMaps() and thus native_init() already
guarantee all these maps have valid file descriptors.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: If89c28923d3ae030dfac0b3b0ab8b15117db3f30