Remove postcss-px-to-rem from Navigation

This was initially discussed in Slack and got the +1 from @nod_, @finnsky and @starshaped. @mherchel had concerns on how this could affect the current themes from core using it, like Claro and Olivero, because the dependencies wouldn't match:

The thing is that we have postcss-px-to-rem in core
And since this is going into core, we want the dependencies to match, right?
If we yank it out of navigation, we'll need to somehow accommodate it when adding it into core. That might not be too easy

I personally don't think this be any problem since we're working on a module. Plus in my opinion, the least postcss plugins we use the better.

Probably we need to add better explanations here.

I think we have to keep rems.

Themes and modules can conrol everything by overriding --admin-toolbar-rem variable, no?

Can be related