Remove the merge driver with the given name.
Attempting to remove the builtin libgit2 merge drivers is not permitted and will return an error.
Currently the merge driver registry is not thread safe, so any registering or deregistering of drivers must be done outside of any possible usage of the drivers (i.e. during application setup or shutdown).
The name under which the merge driver was registered
0 on success, error code < 0 on failure