graphqlbackend: remove DiffSearchResult and DeploymentConfiguration

Created by: keegancsmith

Both types are unused. They aren't the child of any other type, there are no resolvers implemented for them and there are no references to them in the sourcegraph repos outside of the schema. The last point was validated with a query like

repo:github.com/sourcegraph \bVirtualFile\b

on sourcegraph.com.

Subset of https://github.com/sourcegraph/sourcegraph/pull/14326