Remove read_only from options. (#6513)

Remove read_only from options. (#6513)

This PR removes the possibility of declaring options read_only.
It's only effect is making an attempts to disallow changing the respective option from within our internal code (by not providing a setter method). However, a "read-only" option can still be set via the setOption() methods that is also used by the API, and by SMT-LIB's set-option.