Cleanup oxide — Part #1

[RobinMalfait]

This PR is a first step in cleaning up all the oxide related parts in the master branch. The master branch represents v3, all of oxide is in the next branch which represents v4.

This branch still has a lot of crosscheck related checks in the tests but I wanted to do a minimal amount of work to get the repo in a better state. Before this PR we had both the stable and oxide codebase, and we had to swap engines when working on one or the other.

CI also used the swap engines scripts to run the tests against the correct engine. In a previous PR we already dropped all the oxide specific workflows from CI so only the stable engine remains.

This PR does a few things without touching a bajillion amount of test files:

• Swaps the engines by default, so that the stable engine is the default engine instead of the oxide engine.
• Removes all the oxide related code, again just because this is all implemented in v4 which is represented by the next branch.
• Updates CI to not use swap-engines, because we don't need it anymore stable is the default engine.
• Removes the swap-engine.js script.

This PR is a Part 1, the reason being is that all our test files still use a crosscheck function that we used to use to test against both engines. crosscheck is still there but it only ever runs against the stable engine.

The reason we're doing this in two parts is because we want to make the smallest change possible that can be merged without checking a lot of tests files as well.

This is the first step in getting the master branch in a v3-only state instead of a mixed state.

Note: it could be that we get a bunch of Depfu related PRs to bump certain versions. There is a high chance that CI will start failing (especially around major version bumps). We also have to make sure to not bump certain dependencies (double check each failing depfu PR) so that we don't introduce breaking changes in v3.