schannel: not supported with UWP, drop redundant code

[vszakats]

Schannel is not supported by UWP. SSPI is also required by Schannel in
curl, and SSPI also isn't supported by UWP.

mingw-w64 is able to create such build regardless (my guess: due to API
parts not accurately marked as UWP-only), but the binary is unlikely
to work. With MSVC the failure happens at build-time.

Ref: https://learn.microsoft.com/windows/win32/api/sspi/nf-sspi-initsecurityinterfacea#requirements
Ref: https://learn.microsoft.com/windows/win32/secauthn/initializesecuritycontext--schannel#requirements

Drop all UWP-related logic, including two related feature checks, that
can now be permanently enabled.

Also:

• build: show fatal error for Schannel in UWP mode.
• build: do not allow enabling SSPI in UWP mode.
• drop undocumented option DISABLE_SCHANNEL_CLIENT_CERT. Added without
  mention in an unrelated commit. The PR text says to save size. On x64
  this is 0.3%, or 4KB out of 1.3MB. The tiny gain doesn't justify
  an extra build variant. Ref: 8beff43
• move MPROTO_SCHANNEL_CERT_SHARE_KEY closer to its use.
• replace commented block with #if 0.

Reviewed-by: Jay Satiro
Follow-up to cd0ec47 #17089

---

w/o ws https://github.com/curl/curl/pull/18116/files?w=1

• autotools/cmake: add check for the UWP + SSPI + Schannel case.
• temp refix for WinCE? or merge after dropping CE? (→ temp refix)

Tested OK also with VS2008 and WinCE:

• https://ci.appveyor.com/project/curlorg/curl/builds/52492130
• https://github.com/curl/curl/actions/runs/16647643457/job/47112048948?pr=18039

[jay]

Schannel in curl requires SSPI, and SSPI isn't supported by UWP.
Meaning Schannel cannot be used with UWP.

I'm surprised by this. Indeed the doc for InitializeSecurityContext (Schannel) function says in requirements "desktop apps only".

[vszakats]

Schannel in curl requires SSPI, and SSPI isn't supported by UWP.
Meaning Schannel cannot be used with UWP.

I'm surprised by this. Indeed the doc for InitializeSecurityContext (Schannel) function says in requirements "desktop apps only".

Same! Setting aside the false track with mingw-w64, I'm surprised
that UWP apps require a 3rd-party solution for TLS. Perhaps there
is a higher-level protocol facility UWP apps are meant to use.

It'd be nice to actually run the built UWP binaries in CI, to catch similar
issues and this.