Remove usage of extend from table SCSS

What does this MR do and why?

For #323021 (closed)

Hopefully nothing (visually). Removes two usages of @extend from SCSS, and replace them with the actual rules we need. Shaves off around 100 lines from application.css, most of which were not needed. I basically just inspected an actual markdown table and checked which rules were actually being used, and then added those.

Removal of @extend .table deletes the most lines. I think that was originally intended so that markdown tables would look the same as the tables from our own tables.scss. But due to the dark magic of @extends, we also added the selector everywhere that had a .table selector.

So we had bootstrap's dark/light/other table variants:

and their print stylesheets (which I just realised we are probably unwittingly including in gitlab-ui):

We also added our .md table(:not(.code)) selectors to every single bootstrap-vue table rule, since that overrides a number of .table CSS

Screenshots or screen recordings

Here's a table with an image in it, to verify padding, borders, alignment, are the same:

before	after

How to set up and validate locally

1. Create some markdown tables
2. Compare to existing tables on master
3. Observe no changes

I added compiled application.css with before/after to this snippet. The diffs in screenshots were made using FileMerge (macos).

MR acceptance checklist

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