index: don't try to unlink in-memory cache file

Avoid these errors:

Error: unlink((in-memory index).cache) failed: No such file or directory (in mail-cache.c:29)

src/lib-index/mail-cache.c

@@ -25,7 +25,7 @@ void mail_cache_set_syscall_error(struct mail_cache *cache,
 
 static void mail_cache_unlink(struct mail_cache *cache)
 {
-	if (!cache->index->readonly)
+	if (!cache->index->readonly && !MAIL_INDEX_IS_IN_MEMORY(cache->index))
 		i_unlink(cache->filepath);
 }