Don't list build variant release configs

The build variant release configs are not meant to be used in lunch and
therefore shouldn't be listed in ALL_RELEASE_CONFIGS_FOR_PRODUCT.
Otherwise one can do something like "lunch husky-user-user", which
doesn't sound right.

Bug: b/378961230
Test: build/soong/bin/list_releases husky
Change-Id: Ie7a5487aa85018b430c5b816d902149184903eee