pinctrl: lantiq: Remove ltq_pmx_disable() function

Current code adds empty ltq_pmx_disable() because pinmux_check_ops() requires
this callback to be defined.

This is not required since commit 02b50ce4
"pinctrl: make pinmux disable function optional".

Thus remove ltq_pmx_disable() function.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>