Remove `exported_java_libs` from prebuilt_apex/apex_set module types

`exported_java_libs` provide information to soong about which .jars in a
prebuilt .apex need to be deapexed. This property is not used today,
since the only use case for deapexing is for dexpreopt, and dexpreopt is
done using bootjars (encapsulated using
`exported_bootclasspath_fragments`) and systemserver jars (encapsulated
using `exported_systemserverclasspath_fragments`)

Test: go test ./apex
Change-Id: I4ceec74aa11d8e32ccfb0bab29a4489cf8b3fc3d