Remove unnecessary padding in some unit images

Describe the desired feature

Currently the images of Knight and advancements as well as Fire and Skeletal Dragon contain unnecessary transparent pixels to put the unit image at the wanted position relative to the hex they're on. It would be nice to remove these workarounds because they just mean more workarounds for other uses of the unit images where the padding gets in the way (like the image_icon use in those same unit types cropping the padding out again).

There is now support for a [default_frame] tag that can be used to configure the offsets without the padding in the image with its x and y or directional_x and directional_y attributes.

An example can be seen here: https://github.com/wesnoth/wesnoth/blob/f4d6690ae47c72224d7b57697592c5bdf5449240/data/core/units/goblins/Direwolf_Rider.cfg#L6-L11

Would be nice if someone can remove the padding from the images and add a suitable [default_frame] tag to these unit types.