RemapJarTask: Delete output if it exists to prevent ghost files

Fixes FabricMC#1270. This changed isn't necessary for
RemapSourcesJarTask as SourceRemapperService already deletes
the output.

src/main/java/net/fabricmc/loom/task/RemapJarTask.java

@@ -318,6 +318,9 @@ private void remap() throws IOException {
 			Objects.requireNonNull(tinyRemapperService, "tinyRemapperService");
 			Objects.requireNonNull(tinyRemapper, "tinyRemapper");
 
+			// Delete the old file to prevent deleted contents from sticking around in the jar.
+			Files.deleteIfExists(outputFile);
+
 			try (OutputConsumerPath outputConsumer = new OutputConsumerPath.Builder(outputFile).build()) {
 				outputConsumer.addNonClassFiles(inputFile);
 				tinyRemapper.apply(outputConsumer, tinyRemapperService.getOrCreateTag(inputFile));