DRI1 SIGIO handling is non-functional

As noted in the now-withdrawn !555 (closed), since @keithp removed the setting of xf86Info.useSIGIO in commit 6a5a4e60 the code in hw/xfree86/os-support/shared/sigio.c is effectively all dead code now, as it's blocked behind a check for that variable.

The only remaining callers of that code are in hw/xfree86/dri/dri.c, and have apparently been running without SIGIO support since 2015.

If DRI1 doesn't really need this code any more, then the SIGIO code can be deleted in hw/xfree86/dri/dri.c and the sigio.c & sigiostubs.c files can be deleted from os-support/shared and the various os-support Makefiles.

But if DRI does need this code (despite people apparently not noticing since 2015), then a change such as !555 (closed) is needed to make it actually work.