build: drop Windows CE / CeGCC support

Windows CE support was limited to successful builds with ming32ce
(a toolchain that hasn't seen an update since 2009, using an ancient gcc
version and "old mingw"-style SDK headers, that curl deprecated earlier).
Builds with MSVC were broken for a long time. mingw32ce builds were never
actually tested and runtime and unlikely to work due to missing stubs.
Windows CE toolchains also miss to comply with C89. Paired with lack of
demand and support for the platform, curl deprecated it earlier.

This patch removes support from the codebase to ease maintaining Windows
codepaths.

Follow-up to f98c0ba834 #17924
Follow-up to 8491e6574c #17379
Follow-up to 2a292c3984 #15975

Closes #17927

docs/DEPRECATE.md

@@ -12,13 +12,6 @@ email the
 as soon as possible and explain to us why this is a problem for you and
 how your use case cannot be satisfied properly using a workaround.
 
-## Windows CE
-
-Windows CE "mainstream support" ended on October 9, 2018, and "Extended
-Support" ended on October 10, 2023.
-
-curl drops all support in November 2025.
-
 ## VS2008
 
 curl drops support for getting built with Microsoft Visual Studio 2008 in
@@ -100,3 +93,4 @@ Support for RTMP in libcurl gets removed in April 2026.
  - BearSSL (removed in 8.15.0)
  - msh3 (removed in 8.16.0)
  - winbuild build system (removed in 8.17.0)
+ - Windows CE (removed in 8.18.0)