Remove Webjobs.Script.Extensibility dll and nuget package

With BYOB, all extensibility should go through the public WebJobs SDK interfaces and we shouldn't need ScriptBinding classes anymore. Remove all ScriptBinding classes and the Webjobs.Script.Extensibility dll completely.

This is still used for some internal grandfathered bindings. Even while there are still scriptbinding classes, we should be able to remove the nuget package since this is now entirely internal and use the project reference.

Locate the Webjobs.Script.Extensibility project or package references and enumerate the ScriptBinding classes that still depend on them. First inspect the project and dependency graph, then build the host and verify that the package can be removed while the grandfathered internal bindings continue to work.

Written by the indexing model from the issue text.