sgi/install_prerequisites.sh: remove gcc6 from prerequisites.

gcc-6 and g++-6 are not used to build the reference platforms so can be
safely removed from the list of required packages.

Signed-off-by: Sushant Tripathi <sushant.tripathi@arm.com>
Change-Id: Ib3240100795bb7183ce3b2b2c053dd7ba80aa14c