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

java/config/config.go

@@ -90,12 +90,7 @@ func init() {
 		if override := ctx.Config().Getenv("OVERRIDE_JLINK_VERSION_NUMBER"); override != "" {
 			return override
 		}
-		switch ctx.Config().Getenv("EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN") {
-		case "false":
-			return "9"
-		default:
-			return "11"
-		}
+		return "11"
 	})
 
 	pctx.SourcePathVariable("JavaToolchain", "${JavaHome}/bin")