nir/printf: remove treat_doubles_as_floats

It is broken and clang uses fp32 for float constants if the fp64 extension isn't enabled anyway. SPIRVs can't use fp64 constants with printf unless they enable the Float64 cap, which also requires cl_khr_fp64 to be supported.

So just remove it and rely on clang handling -cl-single-precision-constant correctly, which at the moment doesn't seem to be the case, but we can think about that once we plan to support cl_khr_fp64.