Make `bitmap-references' variable permanent

Without this patch all bitmap references are lost on a major mode change.
This fixes one cause of orphaned fringe markers.
Require latest fringe-helper version
Fix fringe overlay clearing

With the latest version of fringe-helper, `fringe-helper-remove' works
as advertised. This fixes the other cause of orphaned fringe markers.
No need to tag fringe overlays

All fringe overlays are already tracked in the `bitmap-references' variable.
Remove unneded init function
Set `git-gutter:window-config-change-function' to nil