build: remove the AUDIO_SUPPORT and DISPLAY_SUPPORT symbols

Nothing tests either symbol any more. AUDIO_SUPPORT was also selected by
PCI_SUPPORT and USB_SUPPORT, which stood for "a sound card could be
plugged in here": audio hardware sits behind MMC, SPI, a Bluetooth UART
or an SoC PCM block just as often, so the inference never held. The
display feature was derived from a literal CONFIG_VGA_CONSOLE or CONFIG_FB
in a kernel fragment, which misses every target whose fbdev came from DRM
emulation.

Delete both symbols, the two selects, the feature translations and the
derivation.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

include/target.mk

@@ -327,9 +327,6 @@ ifeq ($(DUMP),1)
     ifneq ($(CONFIG_PCMCIA)$(CONFIG_PCCARD),)
       FEATURES += pcmcia
     endif
-    ifneq ($(CONFIG_VGA_CONSOLE)$(CONFIG_FB),)
-      FEATURES += display
-    endif
     ifneq ($(CONFIG_RTC_CLASS),)
       FEATURES += rtc
     endif