Remove unnecessary builders.

`delegation.rs` has three builders: `GenericsBuilder`,
`PredicatesBuilder`, and `GenericArgsBuilder`. The first two builders
have just two optional parameters, and the third one has zero. Each
builder is used within a single function. The code is over-engineered.

This commit removes the builders, replacing each with with a single
`build_*` function. This makes the code shorter and simpler.

Author: nnethercote