Revert "Enable Java language level 9 by default."

This reverts commit 848e642096889da02789d81ccfcb454e7c5d8649.

Test: make
Bug: 115604102
Change-Id: I3ce588e89b9c25255d338f3004d1d9876799eb6e
diff --git a/core/config.mk b/core/config.mk
index 84ea03f..d120d61 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -630,11 +630,9 @@
 USE_OPENJDK9 := true
 
 ifeq ($(EXPERIMENTAL_JAVA_LANGUAGE_LEVEL_9),)
-TARGET_OPENJDK9 := true
+TARGET_OPENJDK9 :=
 else ifeq ($(EXPERIMENTAL_JAVA_LANGUAGE_LEVEL_9),true)
 TARGET_OPENJDK9 := true
-else ifeq ($(EXPERIMENTAL_JAVA_LANGUAGE_LEVEL_9),false)
-TARGET_OPENJDK9 :=
 endif
 
 # Path to tools.jar