meson.build: drop config.h.meson

This sits on top of !748 (merged)

Get rid of a number of never/no-longer used #defines and drop the config.h.meson altogeher - meson will produce the correct config.h based on the configuration_data object. Meson's default behavior is to generate a header like this, the main advantage of a config.h.meson header is to filter things that should show up in the config.h file. That comes at the cost of having to #mesondefine every single variable we want.

In most cases, it's easier with meson to just use different configuration_data objects instead - which we do for most of the code.