feat: reduce bytecode by removing unnecessary return values

[TomAFrench]

validateParameters never returns a non-empty reason as it will always revert if an error has occurred. It will also only return in the case where success == true. We then don't need to spend the bytecode associated with returning (true, "") as these return values aren't used.

This removes ~0.09kB of bytecode (giving us plenty of space for #260).

I also noticed that changePoolParameters seems to not be being inlined properly by the compiler. Considering how small the function is you could inline it manually (as you do in _createPool) however I've removed this inlining for consistency as it's a relatively small change in bytecode size.

[Alexangelj]

This looks good - We've been battling contract size so if we can get bytecode reductions elsewhere that would be great, we'd be able to use more runs on the compiler maybe so optimize per call