[ARM] 3301/1: remove unnecessary clock default from ixdp2801 defconfig

Patch from Lennert Buytenhek

The ixdp2x01_clock is already 50MHz by default, so no need to
override it with 50MHz in the ixdp2801 defconfig as is done now,
which is confusing as well.

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