Remove support for EXPERIMENTAL_JAVA_LANGUAGE_LEVEL_9.

Prior to this change, setting that to "false" turned the default Java
language level back to 8. This change removes that option.

Also remove the unused make variable USE_OPENJDK9.

Bug: 115604102
Test: m java droid docs tests cts
Change-Id: I040cf83a2457227ffbcd1c62649e33b98c0c66f7