Drop INDIVIDUALLY_TOGGLEABLE_PREBUILT_MODULES

This list is used to create soong config namespaces per toggle-able
module, and set source_build property. internal and partner branches now
use RELEASE_APEX_CONTRIBUTIONS_* for source vs selection, so remove this
mechanism to prevent confusing behaviors when these two mechanisms have
conflicting values.

`art_module.source_build` is a special-case which uses an additional variable
`ART_MODULE_SOURCE_BUILD`. This will be tracked in
https://r.android.com/3037396

Bug: 308188056
Test: presubmits
Test: downloaded this CL in v-aml branch, m
out/soong/prebuilt_info.json; file is identical

Merged-In: I3f9cbb24d6f3a7a0312683906826708147f469ee
Change-Id: I3f9cbb24d6f3a7a0312683906826708147f469ee