drivers/intel/gma: Ditch link_frequency_270_mhz setting

The `link_frequency_270_mhz` setting was originally used by the native
graphics init code for Sandy/Ivy Bridge, which is long gone.

The value of this information (which board had it set) is questionable.
The only board that had an LVDS panel and set it to 0 was the ThinkPad
L520, where native graphics init was never reported to work. Also, the
native graphics init only used it for calculations, but never confi-
gured the hardware to use a specific frequency. A look into the docu-
mentation also doesn't reveal any straps that could be used to confi-
gure it.

Change-Id: Ieceaa13e4529096a8ba9036479fd84969faebd14
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39763
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Alexander Couzens <lynxis@fe80.eu>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/asrock/b75pro3-m/devicetree.cb

@@ -14,7 +14,6 @@
 #
 
 chip northbridge/intel/sandybridge
-	register "gfx.link_frequency_270_mhz" = "0"
 	register "gfx.use_spread_spectrum_clock" = "0"
 	register "gpu_cpu_backlight" = "0x00000000"
 	register "gpu_dp_b_hotplug" = "4"