Since these expressions were only evaluated immediately, it led to
confusing behavior for users who were expecting that the expression
would be applied per-feature.
Given that expressions can be directly entered into spin boxes, we already
have a way of users evaluating simple calculations for numeric
parameters at least.
I don't think there's a strong enough use case for needing to
calculate string results to leave the confusing expression builder
option in place.
This should be re-evaluated when we add UI support for dynamic
parameters (which are already supported in the backend), where
expressions are evaluated per-feature.
Fixes#17267
