Remove slottable mixin

[Elchi3]

Analog to mdn/browser-compat-data#9048

This time, yarn content move worked as expected. I might have forgotten to run "yarn" from my content folder in the other mixin PR (it is easy to miss).

Anyway, here's another mixin to avoid. For the Text interface it was mostly correct as is already.

I made a few sensible commits which could be preserved (or not, your choice). It didn't make sense to me to put the steps in separate PRs.

[chrisdavidmills]

Again, looking good Mr Scholz.

A few points:

1. Surely /en-US/docs/Web/API/Slottable should redirect to /en-US/docs/Web/API/Element? It currently redirects to nowhere.

2. Same comment as for the other review about possibly providing a note in the spec table to say that the property is in fact defined on a mixin, just in case?

3. Text/assignedSlot shouldn't have the experimental banner at the top of it.

[Elchi3]

Again, looking good Mr Scholz.

A few points:

1. Surely /en-US/docs/Web/API/Slottable should redirect to /en-US/docs/Web/API/Element? It currently redirects to nowhere.

done by doing yarn content add-redirect /en-US/docs/Web/API/Slottable /en-US/docs/Web/API/Element

2. Same comment as for the other review about possibly providing a note in the spec table to say that the property is in fact defined on a mixin, just in case?

Hm, I'm not sure about this one. I think we want to autogenerate the spec section from BCD one day (without comments) and also if you are wondering about mixins you are likely already familiar with the spec?

3. Text/assignedSlot shouldn't have the experimental banner at the top of it.

Removed :)

[chrisdavidmills]

Yay, all looks good to me!

I don't feel that strongly about the mixin note thing; happy to leave that alone. You are right that it would make auto-generation harder.