[media] Cleanup media Kconfig files

- get rid of ridden V4L2_COMMON symbol

  This symbol is not needed anymore; it can be folded with V4L2
  one, simplifying the Kconfig a little bit;

- Comment why some Kconfig items are needed;

- Remove if test for MEDIA_CAMERA_SUPPORT, replacing it by
  depends on.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/i2c/Kconfig

@@ -322,7 +322,7 @@ comment "MPEG video encoders"
 
 config VIDEO_CX2341X
 	tristate "Conexant CX2341x MPEG encoders"
-	depends on VIDEO_V4L2 && VIDEO_V4L2_COMMON
+	depends on VIDEO_V4L2
 	---help---
 	  Support for the Conexant CX23416 MPEG encoders
 	  and CX23415 MPEG encoder/decoders.