Remove custom AnvilInjector indirection previously needed for wrapping MembersInjectors

[kingsleyadio]

At some point before now (well, around Anvil v2.3.3), it was not possible to use MembersInjector directly as the backing system behind Activity (and other framework provided entities) injection. This led us to generate an extra implementation of a custom AnvilInjector interface, which simply delegated back to the original MembersInjector

Now (Anvil v2.3.9), the problem seems to have been fixed as our test clearly shows that using MembersInjector alone is sufficient to complete the graph and provide the necessary system for such entities (activities and the likes), hence the need for our extra indirection via the custom AnvilInjector is no longer necessary and we can safely remove all code around this logic