http2: assortment of small adjustments and clean-up

Because of the speed at which http2 core has been evolving, there's been some accumulation of little quirks and inconsistencies: unused variables, unnecessary variables, strings that should be template strings, etc. This just takes care of some general cleanup and should hopefully — among other things — improve readability a bit.

(Amusingly there's a tiny 2% bump on the http2 headers benchmarks.)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

http2