Source Hubspot: Remove override of `_read_incremental`

[sherifnada]

Tell us about the problem you're trying to solve

In source.py, the hubspot connector overrides the CDK's _read_incremental method because it needs to force a checkpointing of the connector's state. However, with the IncrementalMixin available in the CDK as of this PR we don't need this override logic, and can instead use the mixin to achieve the same result without having to override a private CDK method.

Describe the solution you’d like

Use IncrementalMixin instead of overriding _read_incremental. This is a brittle override which could break if we rename something in the CDK