Drop build support for LEGACY_USE_JAVA7.

This flag allowed for building with an OpenJDK 7 toolchain. It was
used for build bot builds that now work with an OpenJDK 8 toolchain.
Hence, this feature is no longer required. This CL drops it.

Bug: 27583810
Test: Treehugger build succeeds.
Change-Id: I7efca363d665292bfb88f4a4d6f3a120fc8f93f3