Remove fn/closure #[spirv(unroll_loops)] attribute.

[eddyb]

This attribute was pretty hacky, as it affected all loops in the function body, because MIR is a control-flow graph so we can't see "loop expressions" to get attributes from (see below for how we could rectify that!).

To quote the CHANGELOG entry:

• Removed the fn/closure #[spirv(unroll_loops)] attribute, as it has no users,
  is becoming non-trivial to support, and requires redesign for better ergonomics
  (e.g. #[spirv(unroll)] applied to individual loops, not the whole fn/closure)

For more context, see:

• Minimally integrate SPIR-T (opt-in via RUSTGPU_CODEGEN_ARGS=--spirt). #940 (comment)
  (specifically, supporting it on SPIR-T would require additional effort, but there's no users we can find)
• We should override thir_body to inject loop merge points for structurization. #944
  (specifically, the ability to handle loop annotations at a per-loop level, would be massively superior)