Remove the return value from dexpreopt

dexpreopt doesn't strip dex files from the input jar anymore, so there's
no point returning the input jar path untouched.

Test: m nothing + TreeHugger
Change-Id: I8fde6cdc19e85a2fbc946513696417b945c5de0f
diff --git a/java/java.go b/java/java.go
index 02d78f2..9e47b2f 100644
--- a/java/java.go
+++ b/java/java.go
@@ -1786,7 +1786,7 @@
 		j.dexJarFile = dexOutputFile
 
 		// Dexpreopting
-		dexOutputFile = j.dexpreopt(ctx, dexOutputFile)
+		j.dexpreopt(ctx, dexOutputFile)
 
 		j.maybeStrippedDexJarFile = dexOutputFile
 
@@ -3123,7 +3123,7 @@
 
 	j.dexJarFile = dexOutputFile
 
-	dexOutputFile = j.dexpreopt(ctx, dexOutputFile)
+	j.dexpreopt(ctx, dexOutputFile)
 
 	j.maybeStrippedDexJarFile = dexOutputFile