Remove some unused make vars

With https://r.android.com/3199942, the dexpreopt profile file is
installed by s soong module. `DEXPREOPT_IMAGE_PROFILE_BUILT_INSTALLED`
does not need to be exported to make.

Test: no diff in system_intermediates/file_list.txt
Change-Id: I6f7e0648847a5647414f8914ce2b0f2c4737421b