remove committed .index.mf and add to .gitignore

.index.mf is a generated manifest file that should not be tracked in
version control. Remove it from the repo and add it to .gitignore to
prevent accidental re-commits.

.gitignore

@@ -6,5 +6,8 @@
 vendor.tzst
 modcache.tzst
 
+# Generated manifest files
+.index.mf
+
 # Stale files
 .drone.yml