inspector: remove gtk_text_buffer_get_iter_at_line_index() workaround
inspector: remove gtk_text_buffer_get_iter_at_line_index() workaround

The function is now safe to use.

https://bugzilla.gnome.org/show_bug.cgi?id=735341

grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita