Drop cmdgoals special case from prebuilt selection

Before trunk stable, this check ensured that `docs` and `sdk` builds
contained all the apis provided by an apex. If source apex was used, it
would use the source java_sdk_library. If prebuilt apex was used, it
would use the prebuilt java_sdk_library.

After trunk stable, all release configurations for platform development
use the everything stubs of source java_sdk_library for javac, and
exportable stubs of source java_sdk_library for sdk/docs. The exportable
stubs is aware of the enabled flags for a specific release
configuration.

Since sdk and docs builds use the exportable stubs, we can drop this
special case to use the prebuilt stubs/srcjar files.

Test: presubmits
Test: in internal, lunch cf_x86_64_phone-next-userdebug (currently uses
bt prebuilts)
Test: m docs
Test: aninja -t path docs
out/soong/.intermediates/packages/modules/Bluetooth/framework/framework-bluetooth.stubs.source/android_common/exportable/framework-bluetooth.stubs.source-stubs.srcjar;
still exists after this cl

Bug: 308188056

Change-Id: I65da9f25634f0e8c3d514a4c175f89c3e5eb9712