Remove dependencies on Boost.StaticAssert

[Lastique]

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

[coveralls]

coverage: 95.08%. remained the same
when pulling 5e1fe39 on Lastique:feature/remove_static_assert
into fdf2c0f on boostorg:develop.

[Lastique]

@apolukhin Could you take a look at this, please?

[pdimov]

I'm going to merge this because its absence causes errors now that depinst no longer installs static_assert:

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/variant'
error: could not resolve project reference '/boost/static_assert'

That's because boost/static_assert.hpp is in Config now, so its inclusion doesn't introduce a dependency on StaticAssert.