releasetools: Remove two dead functions in ota_from_target_files.py.

GetBuildProp() and GetVendorBuildProp() have been factored into
BuildInfo class. These two functions were meant to be deleted in commit
481bab8d.

Test: python -m unittest test_ota_from_target_files
Test: No users of these two functions.
Change-Id: Id590a46d597939973ef2339cdfea4f5163548d4d