Replace exists() with isDirectory() for consistency

Signed-off-by: Graham <gpe@openrs2.dev>

cache/src/main/java/dev/openrs2/cache/FlatFileStore.kt

@@ -97,7 +97,7 @@ public class FlatFileStore private constructor(
 
     override fun remove(archive: Int) {
         val path = archivePath(archive)
-        if (!Files.exists(path)) {
+        if (!Files.isDirectory(path)) {
             return
         }