platform/sgi575: remove configs specfiying grub config file

The grub config files will be selected by the test case that is being built.
So there is no need to have default grub config files listed the platform
specific build configuration file.

Change-Id: I6a3dd2c41f4b4f95eddb1019e0a77194305e5804
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>