Remove leftover support for target-as-host

Support for target-as-host was removed by 4626b7105763 ("Fix win_sdk
build."), so ADB_HOST_ON_TARGET is never defined.

Test: build
Change-Id: I5ab27e68208677266e2e4272bc2bae7d6a097c5d
diff --git a/adb/adb.h b/adb/adb.h
index e7fcc91..7f7dd0d 100644
--- a/adb/adb.h
+++ b/adb/adb.h
@@ -185,14 +185,7 @@
     } while (0)
 #endif
 
-#if ADB_HOST_ON_TARGET
-/* adb and adbd are coexisting on the target, so use 5038 for adb
- * to avoid conflicting with adbd's usage of 5037
- */
-#define DEFAULT_ADB_PORT 5038
-#else
 #define DEFAULT_ADB_PORT 5037
-#endif
 
 #define DEFAULT_ADB_LOCAL_TRANSPORT_PORT 5555