Always remove image dimensions from markdown during image processing

[rtibbles]

Summary

• I mistakenly believed that Perseus' markdown processing handled the image dimension addition we had made to the markdown, whereas in fact this only gets used in our backend and nowhere else.
• Fixes this by adding a regression test that this is always removed, and simplifying our image replacement behaviour to always do this (and not just for QTI export, as my previous fix had done: Fix image export in QTI exercise publishing #5316)
• Cleans up the previous special casing for this, and simplifies our image replacement flow to just do a single pass using re.sub.

References

Fixes #5398

Reviewer guidance

Do the changes make sense, is the existing test coverage adequate to guard against regressions, is the regression coverage added in this PR adequate to prevent this in future?

[akolson]

Code changes make sense to me! Test should be able to guard against any regressions. Thanks @rtibbles