Eliminate unnecessary Appveyor jobs

[jeking3]

boost::unordered appveyor jobs are really long, consuming about 4 hours of build time, for example:
https://ci.appveyor.com/project/cppalliance/unordered/builds/51261314

consider reducing dependence on appveyor and using more GitHub Actions jobs which allows for more parallelism
The CMT repositories use Boost.CI and have been pruned down to 4 AppVeyor jobs per build.

It looks like this repository is tracking Boost.CI fairly closely. Some of the existing .appveyor.yml configurations may be redundant to the GHA jobs.