Remove __multicall__ support

[goodboy]

Now that #23 has been resolved via #58 we can move towards removing the old _LegacyMultiCall and remaining restrictions surrounding it's support in the core (mostly the _HookCaller afaict).

I was hoping to remove support in a pre-1.0 version and coordinating with pytest appropriately.

[goodboy]

@nicoddemus @RonnyPfannschmidt do you guys think this can be done now?
I'd like to start stripping out old stuff before adding new stuff - call it my OCD.

[RonnyPfannschmidt]

@tgoodlet from a pytest standpoint, this cant be removed before pytest 4.0

[nicoddemus]

Yeah, if we really want to remove it from pluggy, we will probably need to pin down pluggy in pytest or vendor it.

[goodboy]

Heh, yeah let's not do that again.
I'll hold off then.