Always use OpenJDK9 for building

Remove support for compiling with javac from OpenJDK8.
We still target 1.8 by default, and OpenJDK8 prebuilts are still
required for the bootclasspath and running robolectric.

Bug: 38418220
Test: m java
Change-Id: I6f178f73b42331494fadcc25a62a5475656b873e
diff --git a/core/config.mk b/core/config.mk
index 08147cf..53ee359 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -729,17 +729,13 @@
 
 COLUMN:= column
 
-ifeq ($(EXPERIMENTAL_USE_OPENJDK9),)
 USE_OPENJDK9 := true
-TARGET_OPENJDK9 :=
-else ifeq ($(EXPERIMENTAL_USE_OPENJDK9),false)
-USE_OPENJDK9 :=
+
+ifeq ($(EXPERIMENTAL_USE_OPENJDK9),)
 TARGET_OPENJDK9 :=
 else ifeq ($(EXPERIMENTAL_USE_OPENJDK9),1.8)
-USE_OPENJDK9 := true
 TARGET_OPENJDK9 :=
 else ifeq ($(EXPERIMENTAL_USE_OPENJDK9),true)
-USE_OPENJDK9 := true
 TARGET_OPENJDK9 := true
 endif