Remove fallback to classes jars from bootclasspath_fragment

Previously, the hidden API processing performed by the
platform_bootclasspath would try and get flag files from the fragments
it references but would fall back to the classes jars. This change
removes that logic as it is no longer needed as all fragments, both
source and prebuilts, provide flag files and the prebuilts no longer
provide valid classes jars.

Bug: 194063708
Test: m out/soong/hiddenapi/hiddenapi-flags.csv
      - check that this does not change the contents.
Change-Id: Ib867a08508f5a0f8858f6baedebbe99b6a825f80