Remove Column::hasDefaultValue()

This is a follow-up to DBZ-3966; we should not need Column::hasDefaultValue() any longer, as the presence or absence of a column default value can be concluded by examining the Optional returned from defaultValueExpression() (it was needed before, as the actual default value (instead of string-based expression) was stored in ColumnImpl, and we could not distinguish between no default value given at all, or a default value of null being given.

There's a small impact on compatibility though, which is why we probably should keep this change for Debezium 2.0: in some cases, we expose an "implicit default value" such as 0, also if this has not being declared explicitly for that column. The impact of removing hasDefaultValue() can be seen in this PR. Also see the discussions in this PR for more context.