[ARM] 4407/1: Remove in-kernel mach id setting for gtwx5715 ixp4xx board

If MACH_GTWX5715 is set in Kconfig, this code sets the mach id
automatically. Howeber, this means that any IXP4xx kernel which
is setup to support the gtwx5715 board will not successfully boot
on any other board.

If the bootloader sets the wrong mach id, it should be set correctly
by a kernel shim.

Signed-off-by: Michael-Luke Jones <mlj28@cam.ac.uk>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>