Refactor: remove `bmqstm` package

[678098]

bmqstm contains only xsd schema for stat context update/configuration and autogenerated cpp/h files.

There are 2 issues with this:

1. We never actually read stat context configuration from files. It is not necessary to support schema types for it, can be replaced with simple standalone classes.
2. Not necessary to have an entire package to keep it. We can just move 3 files from bmqstm to bmqst.