Remove grf_required_api_level

As we don't fix the grf window, we may not calculate the grf
expiration date and the required api level.
The verification of this will be covered by the tests at run time.

Bug: 176950752
Test: atest --host post_process_props_unittest
Change-Id: I1205f0d9a9da5bc508a49acbcbb7da581800bf45