tcg_interpreter is never written, it is purely a meson option;
trace_backends is never read.

And SeaBIOS is only build from the source tree with roms/Makefile,
so the config.mak file is unused.

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