Clean up the special cases where we wouldn't be able to use the

prebuilt ART module.

They were added with the intention of flipping the last fallback to use
prebuilts by default, but that has been descoped. Instead let the ART
module variable default to the generic MODULE_BUILD_FROM_SOURCE.

Test: env MODULE_BUILD_FROM_SOURCE=true \
        build/soong/soong_ui.bash --dumpvars-mode \
        --vars="MODULE_BUILD_FROM_SOURCE ART_MODULE_BUILD_FROM_SOURCE"
  MODULE_BUILD_FROM_SOURCE='true'
  ART_MODULE_BUILD_FROM_SOURCE='true'
Test: env MODULE_BUILD_FROM_SOURCE=false \
        build/soong/soong_ui.bash --dumpvars-mode \
        --vars="MODULE_BUILD_FROM_SOURCE ART_MODULE_BUILD_FROM_SOURCE"
  MODULE_BUILD_FROM_SOURCE='false'
  ART_MODULE_BUILD_FROM_SOURCE='false'
Test: env MODULE_BUILD_FROM_SOURCE=false \
          ART_MODULE_BUILD_FROM_SOURCE=true \
        build/soong/soong_ui.bash --dumpvars-mode \
        --vars="MODULE_BUILD_FROM_SOURCE ART_MODULE_BUILD_FROM_SOURCE"
  MODULE_BUILD_FROM_SOURCE='false'
  ART_MODULE_BUILD_FROM_SOURCE='true'
Bug: 172480615
Bug: 184842499
Change-Id: I433e0236e595422f4d6fe2b4b2411491e3c600cf