MIPS: defconfig: Cleanup from non-existing options

Remove options which do not exist anymore:
 - CPU_FREQ_DEBUG is gone since commit 2d06d8c4  ("[CPUFREQ] use
   dynamic debug instead of custom infrastructure").

 - 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>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/16770/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>