Remove captive portal code that has no effect.

Note that this CL does not change any behaviour.

At the center of this change is
CaptivePortalTracker#detectCaptivePortal(), which does nothing
except call back into ConnectivityService. Removing it allows us to
simplify code in ConnectivityService. It also allows us to remove
ConnectivityService#captivePortalCheckComplete which was only ever
called in response to this method.

While this does not change any behaviour, it preserves existing
bad behaviour, i.e, that the CAPTIVE_PORTAL_CHECK NetworkInfo
state does not correspond to actual captive portal detection.
We transition into that state and immediately (and unconditionally)
out of it and into CONNECTED.

Change-Id: Ib3797f956d2db5e3cacaaa53e899d81aa8e958af