Run ErrorProne in OpenJDK9

Use the OpenJDK9 javac to run ErrorProne by adding it to
-processorpath and using -Xplugin:ErrorProne.

Bug: 69485063
Test: m RUN_ERROR_PRONE=true javac-check
Change-Id: I0496006b71b70766ef16d57753cbcf037897799c
diff --git a/ui/build/config.go b/ui/build/config.go
index 345d8ea..5dcdf87 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -165,15 +165,7 @@
 			return override
 		}
 		v, ok := ret.environ.Get("EXPERIMENTAL_USE_OPENJDK9")
-		if !ok {
-			v2, ok2 := ret.environ.Get("RUN_ERROR_PRONE")
-			if ok2 && (v2 == "true") {
-				v = "false"
-			} else {
-				v = "1.8"
-			}
-		}
-		if v != "false" {
+		if !ok || v != "false" {
 			return java9Home
 		}
 		return java8Home