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

lib/curlx/fopen.h

@@ -34,7 +34,7 @@
 
 int curlx_fseek(void *stream, curl_off_t offset, int whence);
 
-#if defined(_WIN32) && !defined(UNDER_CE)
+#ifdef _WIN32
 FILE *curlx_win32_fopen(const char *filename, const char *mode);
 int curlx_win32_stat(const char *path, struct_stat *buffer);
 int curlx_win32_open(const char *filename, int oflag, ...);