schannel: drop fallbacks for 4 macros

[vszakats]

They are defined by all mingw-w64 versions and all supported MSVC
versions (SDK 7.1A+).

Also by OpenWatcom 2:
https://github.com/open-watcom/open-watcom-v2/blob/ce6c37eb29f3fda95f9c4e8e37dee866b8c4e496/bld/w32api/include/wincrypt.mh

These aren't defined by mingw32ce. And likely defined by MS WinCE SDK,
but curl code doesn't use them in WinCE builds.

[vszakats]

This may have been premature and/or the commit message inaccurate.
VS2008 came with MS SDK 6.0a, which is what we support now, and
VS2010 came with MS SDK 7.0a, which is soon to be the minimum.
I need to get a copy of these old versions and recalculate.

edit: we do test with VS2010 in CI, so that should be fine.

[vszakats]

Passed VS2008 in CI: https://ci.appveyor.com/project/curlorg/curl/builds/52486233/job/ptntb3r9f5n5n4yy

[vszakats]

Confirmed that all 4 macros are present in MS SDK 6.0A (VS2008) and later.