Remove filter effect on speech bubbles (fixes #110)

Some (parts of the) speech bubbles had a filter applied, and this caused them to be rasterised when exporting to PDF, leading to low-quality output. The actual filter (‘filter4386’) was only defined in the files for episode 1, so they didn’t have an effect on the rendering (except rasterisation) for later episodes. And even for episode 1, they don’t have any visible effect, so they are removed for all episodes.