Remove the EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN option.

Previously, this setting environment variable to false reverted to an
OpenJDK 9 toolchain. This change removes that option.

Test: m core-all-system-modules
Test: zcat out/verbose.log.gz | grep 'prebuilts/jdk/jdk[0-9]*'
Test: EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN=false m core-all-system-modules (fails)
Bug: 131683177
Change-Id: I0d679648e1236925ce3ef7e1652379127e846b00
diff --git a/ui/build/config.go b/ui/build/config.go
index 686ca3e..92aee4d 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -219,8 +219,8 @@
 		if override, ok := ret.environ.Get("OVERRIDE_ANDROID_JAVA_HOME"); ok {
 			return override
 		}
-		if toolchain9, ok := ret.environ.Get("EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN"); ok && toolchain9 == "false" {
-			return java9Home
+		if toolchain11, ok := ret.environ.Get("EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN"); ok && toolchain11 != "true" {
+			ctx.Fatalln("The environment variable EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN is no longer supported. An OpenJDK 11 toolchain is now the global default.")
 		}
 		return java11Home
 	}()