mb/google/myst: Remove PRESERVE FMD flag for RW_MRC_CACHE

The PRESERVE flag in the FMD file tells futility not to erase the
fmap partition when updating the firmware.  Because of an issue on
myst right now, we want the RW_MRC_CACHE partition to be erased
when the firmware is updated.

BUG=b:290763369
TEST=None

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: Id586ae057b2fd6d513ddbba5e1284dea39467d95
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76478
Reviewed-by: Jason Glenesk <jason.glenesk@amd.corp-partner.google.com>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Jon Murphy <jpmurphy@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>

src/mainboard/google/myst/chromeos.fmd

@@ -33,7 +33,7 @@ FLASH@0xFF000000 16M {
 		SMMSTORE(PRESERVE) 64K
 		RW_LEGACY(CBFS)
 		RW_VBIOS_CACHE 64K
-		RW_MRC_CACHE(PRESERVE) 256K
+		RW_MRC_CACHE 256K
 		RECOVERY_MRC_CACHE(PRESERVE) 256K
 	}
 }