nir/format_convert: remove unorm bit size assert
Yes, we're losing precision if this assert fails and it's wrong. It's also necessary to implement GL in a reasonable way on Asahi. Remove the assert that was recently added and add more comment context on the mess.
Fixes debug build regression on asahi:
dEQP-GLES3.functional.vertex_arrays.single_attribute.normalize.int.components4_quads1