remove the `usize` field from `CandidateSource::AliasBound`

[lcnr]

The field is currently unused and can be incorrect due to the way the new solver deals with projections

rust/compiler/rustc_trait_selection/src/solve/assembly.rs

Line 79 in 7919ef0

AliasBound(usize),

rust/compiler/rustc_trait_selection/src/solve/assembly.rs

Lines 220 to 221 in 7919ef0

	// FIXME: This is broken if we care about the `usize` of `AliasBound` because the self type
	// could be normalized to yet another projection with different item bounds.

[lenko-d]

@rustbot claim