extensions-tool: Fix removing from settings list

extensions-tool: Fix removing from settings list

When removing a string from a settings list, we iterate over all
existing entries and copy all strings except the one that's being
removed to a new list, which is then written to GSettings.

However we currently always increment the index, so we end up with
a NULL entry in place of the removed entry, which is then interpreted
as the end of the list. In other words, we also remove all entries
that follow the removed string.

Fix this by looping over the list entries instead of the index, and
only increment the index for entries we copy.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1946