remove persist.sys.usb.config override

The variable persist.sys.usb.config is no longer honored by the framework,
and it's presence causes unnecessary toggling of the USB driver, which
disonnects ADB and makes the device's connection unstable.

Delete it.

Bug: 21404762
Bug: 18905620
Change-Id: Iaeded8a7617e9d787480b91387ca993d4e0a786c

device.mk

@@ -28,10 +28,6 @@ PRODUCT_COPY_FILES += \
     frameworks/native/data/etc/android.hardware.sensor.proximity.xml:system/etc/permissions/android.hardware.sensor.proximity.xml \
     frameworks/native/data/etc/handheld_core_hardware.xml:system/etc/permissions/handheld_core_hardware.xml
 
-# System Properties
-PRODUCT_DEFAULT_PROPERTY_OVERRIDES += \
-    persist.sys.usb.config=mtp
-
 # Screen density
 PRODUCT_AAPT_CONFIG := normal hdpi xhdpi
 PRODUCT_AAPT_PREF_CONFIG := xhdpi