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
diff --git a/services.cpp b/services.cpp
index 63a0a76..68545d8 100644
--- a/services.cpp
+++ b/services.cpp
@@ -465,9 +465,9 @@
         ret = create_service_thread(framebuffer_service, 0);
     } else if (!strncmp(name, "jdwp:", 5)) {
         ret = create_jdwp_connection_fd(atoi(name+5));
-    } else if(!HOST && !strncmp(name, "shell:", 6)) {
+    } else if(!strncmp(name, "shell:", 6)) {
         ret = create_subproc_thread(name + 6, true);
-    } else if(!HOST && !strncmp(name, "exec:", 5)) {
+    } else if(!strncmp(name, "exec:", 5)) {
         ret = create_subproc_thread(name + 5);
     } else if(!strncmp(name, "sync:", 5)) {
         ret = create_service_thread(file_sync_service, NULL);