stubs: Remove unused arch_type.c stub

stubs: Remove unused arch_type.c stub

We added a stub for the arch_type global in commit 5964ed56d9a1 so
that we could compile blockdev.c into the tools.  However, in commit
9db1d3a2be9bf we removed the only use of arch_type from blockdev.c.
The stub is therefore no longer needed, and we can delete it again,
together with the QEMU_ARCH_NONE value that only the stub was using.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210730105947.28215-9-peter.maydell@linaro.org