Remove bound_vars field of CallableType

[rwbarton]

This field was effectively unused since the removal of the runtime
type checking target in commit 6dca530 "Remove obsolete
transform-related code".

[gvanrossum]

I love the smell of deleted code in the morning!

Interesting that the last remaining use was in __repr__() of CallableType.

[gnprice]

Nice diffstat. :-)

It looks like a couple of these methods still indicate in their docstrings that they do something with these -- probably good to cut those too so they don't confuse someone in the future who doesn't know what an "implicit bound type" is supposed to be.

$ git grep -n implicit.*bound
mypy/applytype.py:15:    'def [-1:int] (int) -> int'. Here '[-1:int]' is an implicit bound type
mypy/types.py:936:     - Include implicit bound type variables of callables.

[rwbarton]

Good point Greg. I filed #1385.