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
diff --git a/core/java_common.mk b/core/java_common.mk
index dfe75f3..f6e01d2 100644
--- a/core/java_common.mk
+++ b/core/java_common.mk
@@ -33,8 +33,7 @@
     # TODO(ccross): allow 1.9 for current and unbundled once we have SDK system modules
     LOCAL_JAVA_LANGUAGE_VERSION := 1.8
   else
-    # DEFAULT_JAVA_LANGUAGE_VERSION is 1.8, unless TARGET_OPENJDK9 in which case it is 1.9
-    LOCAL_JAVA_LANGUAGE_VERSION := $(DEFAULT_JAVA_LANGUAGE_VERSION)
+    LOCAL_JAVA_LANGUAGE_VERSION := 1.9
   endif
 endif
 LOCAL_JAVACFLAGS += -source $(LOCAL_JAVA_LANGUAGE_VERSION) -target $(LOCAL_JAVA_LANGUAGE_VERSION)