Remove confusing variable HOST.

First, HOST is always 0 in adbd, which matches ADB_HOST=0.
Second, HOST is always 1 when adb_main is called, which matches ADB_HOST=1.
For adb client that doesn't call adb_main, it never touches local_init(),
init_transport_registration() and fdevent_loop(). So the changes in adb.cpp,
services.cpp and transport_local.cpp do nothing with it.
As a conclusion, I think we can remove HOST and use ADB_HOST instead.

Change-Id: Ide0e0eca7468b6c3c130f6b50974406280678b2e