refactor and remove GlobalSettings.kem

**please review**
Those steps are moved to `KompileBackend`. I did this change in the past but hesitated to send a pull request for some reason. For the motivation, see this chart:

Backend name         |  Overrides BasicBackend steps
----------------------------------------------------
KompileBackend       |  yes
JavaSymbolicBackend  |  yes
CoqBackend           |  yes
HtmlBackend          |  no
LatexBackend         |  no
PdfBackend           |  no
UnparserBackend      |  no

The thing to observe here is that all backends that are meant to execute the code already overrides it, and the ones that don't override actually don't need any steps at all.

So default steps provided by `BasicBackend` was not really a good default.

(Another advantage of moving steps out of `BasicBackend` is performance, because now `HtmlBackend`, `LatexBackend`, `PdfBackend` and `UnparserBackend` won't be running unnecessary steps, although the cost of the steps were probably very small)