Remove dependency on ImageMagick convert command

[rbeezer]

As of #2186 the convert executable is only needed for the gauntlet of EPUB auto-generated covers.

Can we coax pyMuPDF into doing the job? Or something else we already use? Or skip that attempt entirely?

@oscarlevin might be interested collaterally.

[Alex-Jordan]

The gauntlet was a fun exercise, but I'm fine with simplifying things to either (a) you provide the image yourself or (b) there is one fallback with "the best" chance of working for all users. Is it worth keeping the gauntlet or going with just one fallback? (I guess then it's a hard question as to which thing should be the fallback...)

[rbeezer]

One fallback would be great. A balance of functionality and likelihood of success?