Dedup art-bootclasspath-fragment contents from sdk

Previously, the sdk duplicated the modules from the contents list of
the art-bootclasspath-fragment. That is no longer necessary as the
contents of bootclasspath_fragment modules are automatically added to
the sdk.

Bug: 177892522
Test: m art-module-sdk
      - check generated snapshot still contains core-oj, core-libart,
        etc.
Change-Id: I1a8720ff41981f31595a5d2d30164afd0a3a5764