Remove dependencies on Boost.StaticAssert

[Lastique]

Boost.StaticAssert has been merged into Boost.Config, so remove the dependency.

[pdimov]

I'm going to merge this, because without it we get errors like

error: Unable to find file or target named
error:     '/boost/static_assert//boost_static_assert'
error: referred to from project at
error:     '/home/runner/work/boost_install/boost-root/libs/python/build'
error: could not resolve project reference '/boost/static_assert'

now.

[pdimov]

This PR needs to go into master as well, but since there are nontrivial commits on develop, I won't merge it. Please take care of that.

[stefanseefeld]

I will have a look in the next couple of days. Thanks.