4.5.0 - Update for the nested subpath changes in FluentKit
This patch was authored and released by @gwynne.
vapor/fluent-kit#572 takes advantage of the new support added to SQLKit in vapor/sql-kit#169, so remove the obsolete legacy support here.
Also does some minor tidying.
(Note: Since we now depend on a version of FluentKit that is multiple minor versions newer than before, this must also be semver-minor.) #87