Can we deprecate the side input paths through PAssert?

PAssert has two distinct paths - one uses GBK with a single-firing trigger, and one uses side inputs. Side inputs are usually a later addition to a runner, while GBK is one of the first primitives (with a single firing it is even simple). Filing this against myself to figure out why the side input version is not deprecated, and if it can be deprecated.

Marking this as a "starter" task because finding and eliminating side input version of PAssert should be fairly easy. You might need help but can ask on dev@

Imported from Jira [BEAM-6257](https://issues.apache.org/jira/browse/BEAM-6257). Original Jira may contain additional context.
Reported by: kenn.