manager/poetry.lock: remove

Lockfiles are generally encouraged to be committed into VCS to ensure reproducible builds on all machines. Our situation is slightly different - we are more interested in supporting a wide range of setups on a wide range of systems.

Our poetry.lock also contained some outdated library versions that had CVEs reported on them, and GitHub did not like that, so this incidentally also solves that problem for us. Since the lock file was only used during development, this will not affect end users in any way (runtime dependencies are managed by each distro's package manager).