Remove STL workarounds for VSO-1006185

[Casey Carter (CaseyCarter)]

Description

...which has been fixed for 16.6p1.

Fixes #416.

[This is a dual of internal PR MSVC-PR-222093.]

Checklist

Be sure you've read README.md and understand the scope of this repo.

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• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 or there are no product code changes.
• The STL builds successfully and all tests have passed (must be manually
  verified by an STL maintainer before automated testing is enabled on GitHub,
  leave this unchecked for initial submission).
• These changes introduce no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate
  or trivially copyable, etc.).
• These changes were written from scratch using only this repository,
  the C++ Working Draft (including any cited standards), other WG21 papers
  (excluding reference implementations outside of proposed standard wording),
  and LWG issues as reference material. If they were derived from a project
  that's already listed in NOTICE.txt, that's fine, but please mention it.
  If they were derived from any other project (including Boost and libc++,
  which are not yet listed in NOTICE.txt), you must mention it here,
  so we can determine whether the license is compatible and what else needs
  to be done.

[Casey Carter (CaseyCarter)]

Blocked on release of Visual Studio 2019 16.6 Preview 1.

[Stephan T. Lavavej (StephanTLavavej)]

Looks good to me. Remember that there are workarounds in devcrt too (3 occurrences in Dev11_1140665_unique_ptr_array_conversions); depending on when we can migrate devcrt to GitHub, you'll either need to update this PR, or add Microsoft-internal test changes.