docker: Remove arm64ec from the system-clang testcase

Building for arm64ec requires Clang/LLD 21.

This was added in cb92de78e7 without
considering the context in this testcase properly.

Dockerfile.system-clang

@@ -22,7 +22,8 @@ WORKDIR /build
 
 ENV TOOLCHAIN_PREFIX=/opt/llvm-mingw
 
-ARG TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64 arm64ec"
+# arm64ec requires Clang/LLD 21
+ARG TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64"
 
 ARG DEFAULT_CRT=ucrt