Stop shipping package 'default-libmysqld-dev'

MySQL did not have a 'libmysqld-dev', and starting with MariaDB 11.4 in Debian it does not have it either. No package depends on it, and anyway anyone building an embedded server is most likely not building it using a full Debian system, so assumed user count is very low, and having the package is not worth the effort, so stop shipping it.