hw/display/tcx: Drop unnecessary code for handling BGR format outputs

hw/display/tcx: Drop unnecessary code for handling BGR format outputs

For a long time now the UI layer has guaranteed that the console
surface is always 32 bits per pixel, RGB. The TCX code already
assumes 32bpp, but it still has some checks of is_surface_bgr()
in an attempt to support 32bpp BGR. is_surface_bgr() will always
return false for the qemu_console_surface(), unless the display
device itself has deliberately created an alternate-format
surface via a function like qemu_create_displaysurface_from().

Drop the never-used BGR-handling code, and assert that we have
a 32-bit surface rather than just doing nothing if it isn't.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210215102149.20513-1-peter.maydell@linaro.org