Remove the support for BRILLO_VENDOR_PARTITIONS.

It was initially introduced in commit
2e735ca3, where it packs additional
vendor images into target_files zip in order to generate OTAs. We can
acheive the same goal with INSTALLED_RADIOIMAGE_TARGET, which is the way
being actively used across all targets, including IoT (the former
Brillo) targets.

Bug: 78201540
Test: `m dist` with aosp_marlin-userdebug
Test: Code search shows no active user of BRILLO_VENDOR_PARTITIONS.
Test: `python -m unittest test_add_img_to_target_files`
Change-Id: I8803d5377b5a39304a701cceafb243f9a228347d