Remove GTEST_HAS_STD_STRING

[kuzkry]

1st commit removes GTEST_HAS_STD_STRING. I think we can drop it, after all it's 2019 :)
2nd commit simply removes usings from the header, which is how all guidelines say

Both changes are backward incompatible. I would merge them though because README.md says the following about post 1.8.x releases:

On-going work to improve/cleanup/pay technical debt

and we don't quite need it, right?

[gennadiycivil]

273567737
Thank you, we have started internal review. Please don't push any more changes into this PR as they might be overwritten.

[gennadiycivil]

@kuzkry
So I run some internal tests, GTEST_HAS_STD_STRING looks OK ( I think but not 100% sure because of all of the noise from the other change ).

There is another change in this PR, ::testing::tuple_size -> std::tuple_size. This one looks harder because there is internal code accumulated over the years that is still using ::testing::tuple_size.
May I suggest splitting these two changes into two separate PRs?
Thank you!
G

[kuzkry]

Sure, done.

[gennadiycivil]

Trying again
273993693
Thank you, we have started internal review. Please don't push any more changes into this PR as they might be overwritten.