Don't clean INSTALLED_RADIOIMAGE_TARGET in target/board/Android.mk

We are deprecating use of AndroidBoard.mk. To move add-radio-file to
Android.mk, cleaning INSTALLED_RADIOIMAGE_TARGET may clean the radio
target set by other Android.mks that get included before
target/board/Android.mk.
Bug: 3500660

Change-Id: Iee19c40c18f0e9ec23bad4b6ab0e600756af615a

target/board/Android.mk

@@ -20,9 +20,6 @@ else
   INSTALLED_KERNEL_TARGET :=
 endif
 
-# Use the add-radio-file function to add values to this variable.
-INSTALLED_RADIOIMAGE_TARGET :=
-
 -include $(TARGET_DEVICE_DIR)/AndroidBoard.mk
 
 # Generate a file that contains various information about the