refactor(injector): remove unused code

[pomerantsev]

The removed code isn't being used:

• there isn't a single place in the code where invoke would be
  called with a string argument in the third position.
• the fourth parameter is also not documented,
  so when called from outside of the framework's source,
  the third parameter is meant to be the locals object.

[mary-poppins]

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[caitp]

This is really a convenience for 3rd party libraries which use the smart-injector style for providing more debug information. I don't feel good about breaking that just because we don't use it internally. It's not clear whether anyone is actually using it yet, but they might at some point

[pomerantsev]

Thanks for your quick response!
I've changed the commit according to it.
This was something that tripped me up when I was looking at the source code, so I feel that saying a couple of words about it in the docs won't do any harm.
If you feel otherwise, I'm okay with closing this PR.

[caitp]

well hang on, maybe the original PR isn't a bad idea. I'm thinking about it.

[pomerantsev]

Okay - please let me know.

[al-the-x]

@caitp Can you explain how a 3rd party library might actually use this functionality?

[caitp]

It's just a simpler way to call it, for instance $injector.invoke(someFn, 'DescriptiveNameForWhatSomeFnIs');

Obviously this is in the slot for locals, but it's convenient to be able to call the function with 2 params rather than three (however, I'm not sure if this is really all that great, it might be better to not support the case where locals is a string).

But basically what it does is provides some extra metadata about the service being instantiated or invoked, in order to make it easier to find where the problem lies.

[pomerantsev]

Okay - @caitp - would you like me to remove those first three lines from injector.invoke()?
Personally, I don't feel they add to the API's consistency, but I'm totally leaving this up to you.

[al-the-x]

Ah, as is done in unit testing, typically. Seems like just passing an empty
object literal as the locals dictionary isn't overly painful, though. If
documented, I can see the utility, but having it both ways seems like
over-engineering to me.