The cmd_line.txt mangling is only needed when rebuilding from very old
trees and is kept mostly as an example of how to extend it.  However,
Meson 0.63 introduces a deprecation mechanism for meson_options.txt
that can be used instead, so get rid of our home-grown hack.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>