Cleanup matcher_execution_context.

* The external @matcher_execution_context (from `self`
  at the point the matcher method was called) was odd,
  and turns out never to have been needed.  It was
  introduced in abba43932a821b3c7f60edfe247ee1917dbf1dac
  but wasn't actually needed there (as the spec added
  in that commit passed when I tried removing it).
* Given that it must be set (as `respond_to?` assumes
  it has been set), it was odd to have an extra
  attribute that gets set after instantiation.
  Far better to make it a constructor arg.
* No need to expose it as public method.
Remove `RSpec::Matchers::Pretty#_pretty_print`.

It's nearly identical to `to_sentence`.
Remove expected_to_sentence method.

It's unnecessary and has some odd coupling, where it
assumes the including class has an `expected` attribute.

I prefer the simplicity of `to_sentence expected`.
Merge pull request #485 from rspec/remove-unneeded-ivar

Cleanup matcher_execution_context.