luci-app-acl: unset read and write before acl set

If the setting in the view is set to `denied`, only the read list option
is deleted. This is not correct. The write list option must also be
deleted. To ensure that the correct configuration is saved, the write
and read list options are always deleted beforehand and then rewritten.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>