Cleaning up praefect method signatures

This MR cleans up praefect method signatures. Mainly, it removes some parameters that aren't necessary. There are no functional changes in this MR.

This MR is in preparation for adding https://gitlab.com/gitlab-org/gitaly/-/blob/master/internal/praefect/middleware/methodtype.go, for which we will need a registry to be passed down into the server constructors.