Drop some special casing of source vs prebuilt mainline selection

soong config vars are no longer needed to support the following two
special cases
1. Santized builds. google hwasan products have been updated to explicitly list
   PRODUCT_MODULE_BUILD_FROM_SOURCE ag/26576335
2. Coverage builds. product specific overrides of build flags are not
   possible, so apex_contributions.go has been special cased to ignore
   prebuilts. Since RELEASE_APEX_CONTRIBUTIONS_* are now used to select
   prebuilts of mainline modules, we can drop the special-case of
   coverage builds in this .mk file

Bug: 308188056
Bug: 325666427
Test: lunch cf_x86_64_phone-next-userdebug && EMMA_INSTRUMENT=true m
nothing; # verified that no path exists between droid and the prebuilt
bt apex

Change-Id: Ide50e4463615fef5f540130f6a6bce50c00eddb8