ARM: ixp4xx_defconfig: Cleanup from non-existing options

Remove options which do not exist anymore:
 - ECONET is gone since commit 349f29d8 ("econet: remove ancient bug
   ridden protocol");

 - IPDDP_DECAP is gone since commit 9b5645b5 ("appletalk: remove
   "config IPDDP_DECAP"");

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>