ref(grouping): Change where message variable is parameterized

[lobsterkatie]

This is a small refactor to the code used for variable resolution in custom fingerprints. Recently, we began parameterizing the {{ message }} variable using normalize_message_for_grouping inside of resolve_fingerprint_variable, but subsequently realized we needed to prevent parameterization when the variable is used for a custom title. Thus we introduced a parameterize_message parameter to resolve_fingerprint_variable, to control whether or not parameterization runs.

This PR is a different (better) solution to the same problem, done in order to both simplify the code and decrease the blast radius of an upcoming change to normalize_message_for_grouping. Instead of having parameterize_message: True at one of the two resolve_fingerprint_variable call sites and parameterize_message: False at the other, it just moves the parameterization to the one place where parameterize_message is currently true. This lets us get rid of the new parameter, and prevents the parameterize_message: False code from having to compensate for the normalize_message_for_grouping changes.