Eliminate float printing via mp_printf?

[jepler]

Reference:  checks: Add checking of vstr_printf, mp_warning, DEBUG_printf. [e9c3444368]
Comparison: core: Make printf 'f' format selectable at build time. [537e893162]
   unix x64:  -192 -0.020% standard
      stm32:  -176 -0.043% PYBV10

It'd save 100+ bytes on some common ports. And with the work in progress compiler plugin (#17556) we can have some confidence there are no uses of it anyway.