test(win32): drop the __libc_single_threaded stand-in, which crafter-build 5.0.0 made unneeded

Tests
- Win32Link linked under the Flatpak only with a C file defining
  __libc_single_threaded. crafter-build gave clang the host's /usr as
  the MinGW sysroot, which put glibc's headers on the include path, and
  libstdc++'s shared_ptr, inlined from nlohmann/json in Crafter.Asset's
  GLB loader, then read the symbol. crafter-build 5.0.0 shows clang only
  the mingw-w64 parts of /usr (Catcrafts/Crafter.Build#43), so nothing
  references it any more. Under 5.0.0 Win32Link links without it, and so
  does examples/HelloUI, which no stand-in covered. The native
  crafter-build on the host, which never had the problem, still builds
  PointerWin32, TextWin32 and Win32Link and runs them under Wine.

Docs
- AGENTS.template.md and AGENTS.consumer.md: linking for Win32 under the
  Flatpak needs crafter-build 5.0.0, and the workaround is gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/Win32Link/libc-single-threaded.c

@@ -1,11 +0,0 @@
-//SPDX-License-Identifier: LGPL-3.0-only
-//SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
-
-// Stands in for Crafter.Build #43, and goes when that is fixed, with its line
-// in project.cpp. In the Flatpak the MinGW sysroot is the host's /usr, which
-// puts glibc's headers on the include path, so the libstdc++ shared_ptr that
-// nlohmann/json inlines into Crafter.Asset's GLB loader reads glibc's
-// __libc_single_threaded, which no Windows library defines. Zero means "not
-// single-threaded", so the reference counts stay atomic, as they would be
-// without the leak.
-char __libc_single_threaded = 0;