Revert adding cancel package to default environment.

[SeanESCA]

As per my comment in #271, this PR removes the cancel package from the default environment. Additionally, it removes checking for the cancel package in the TeX writer.

[jgm]

Btw, I've altered pandoc's LaTeX writer so that it sets a cancel variable if any of the cancel macros are used in math; that triggers conditional inclusion of \usepackage{cancel}.

[SeanESCA]

Btw, I've altered pandoc's LaTeX writer so that it sets a cancel variable if any of the cancel macros are used in math; that triggers conditional inclusion of \usepackage{cancel}.

Thanks @jgm! Could we do something similar for the siunitx package? It looks like you still have to add it yourself for TeX and PDF although texmath supports it.

[jgm]

In theory yes, but there are so many siunitx commands that it would be rather hard to test for them.
Prefer to keep things simple.