Remove -Werror=double-promotion

[Daft-Freak]

This has been here ~forever because of "serious performance implications", but ends up causing a bunch of other issues. Like making it really annoying to print a float:

error: implicit conversion from 'float' to 'double' when passing argument to function [-Werror=double-promotion]
   32 |     snprintf(buf, sizeof(buf), "Accel\nX: %f\nY: %f\nZ: %f", accel_val.x, accel_val.y, accel_val.z);
      |                                                              ~~~~~~~~~~^

Also, I'm not sure how much -fsingle-precision-constant helps considering that it makes the behaviour different depending on what you're building for...

auto d = 1.0; // sometimes a float, sometimes a double!

[Gadgetoid]

I only have the very deepest darkest recollection of these!