[ARM] 3081/1: Remove GTWX5715 from ixp4xx_defconfig

Patch from Deepak Saxena

CONFIG_MACH_GTWX5715 hardcodes the machine type in head-xscale.S so we
can no longer boot on any other machine types. The proper fix would be
to remove the hardcoding, but that machine is an off-the-shelf system
and most users won't have access to the bootloader. :(

Signed-off-by: Deepak Saxena <dsaxena@plexity.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>