Remove unnecessary special case for zero_t in code gen that caused unused-variable warning in thunks.
Add instructions for tidying warnings (with the present zero_t case as its running example) as a "proposal."
The code gen change is safe because a corresponding case for one_t was always "missing" and the the variable at issue is never used meaningfully. (Prior work has size_t lowered as int. While that's a dubious choice, it's beyond what I'm fixing here.)
Mark these tests as now clean of warnings:
array-collections/array-basic
ctrl-flow/loopctrl
vector
vector_math/vec2_int
vector_math/vec2_float
vector_math/vec3_float
vector_math/vec4_float
zero_one
