Drop ART_MODULE_BUILD_FROM_SOURCE

RELEASE_APEX_CONTRIBUTIONS_ART provides a similar functionality, and
also allows selection of a specific art prebuilt if multiple art
prebuilts exists in the tree

`art_module.source_build` was also previously used to conditionally
disable the source soong modules of art/, but this conditional disabling
has been removed with r.android.com/3045792

Bug: 308188056
Test: presubmits
Test: lunch cf_x86_64_phone-trunk_staging-userdebug; mmma art
Test: git_master-art:art-host-x86_64,art-target-arm64,mainline_modules
https://android-build.corp.google.com/builds/abtd/run/L30200030003163172
Test: git_master-art-host:art-build,art-gtest,art-test,art-preopt
https://android-build.corp.google.com/builds/abtd/run/L07600030003164399

Change-Id: Idbb129d2ec304d5320754cd61d40e10e5c7d5414