ARM: defconfig: remove broken CONFIG_THUMB disables

Since commit 1515b186 ("ARM: make configuration of userspace
Thumb support an expert option"), CONFIG_THUMB cannot be disabled
unless one turns on CONFIG_EXPERT first.

This is probably for the better, so remove the statements that
turn it off.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>