kingst-la2016: remove complex logic threshold voltage support

kingst-la2016: remove complex logic threshold voltage support

Remove the complex implementation of the three config key support to
specify the logic input threshold voltage, as well as the compile time
switch to pick from alternative implementations. This approach is easy
to come back to later when desired, its removal is in a separate VCS
commit. Getting the full -4.0..+4.0V range and continuous adjustment
would be nice to have. But it's essential for users today to see the
currently effective voltage in UIs.