Fix adb logcat dc on boot.

persist.sys.usb.config is set to mtp in device.mk, which
causes adb logcat to disconnect on bootup. Fix by removing this
line.

UsbDeviceManager handles making mtp default when necessary.

Test: adb reboot && adb logcat
Change-Id: Iffdfd6730975aa4c8b0ac17d1bcb665111dc1d52
diff --git a/device.mk b/device.mk
index 8d830c0..61cb2b5 100644
--- a/device.mk
+++ b/device.mk
@@ -219,7 +219,6 @@
 
 # set default USB configuration
 PRODUCT_DEFAULT_PROPERTY_OVERRIDES += \
-    persist.sys.usb.config=mtp \
     ro.adb.secure=1 \
     ro.sf.lcd_density=320 \
     ro.opengles.version=196610