luci-base: form.js: consider aliased options in AbstractValue.remove()

If different options point to the same underlying uci option, we must only
remove the uci value if none of the other alias options is active in order
to prevent inactive options (due to unsatisfied depends) removing the uci
value of active once on save.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>