Rollup merge of #88810 - camelid:cleanup-pt1, r=jyn514

rustdoc: Cleanup `clean` part 1

Split out from #88379.

These commits are completely independent of each other, and each is a fairly
small change (the last few are new commits; they are not from #88379):

- Remove unnecessary `Cache.*_did` fields
- rustdoc: Get symbol for `TyParam` directly
- Create a valid `Res` in `external_path()`
- Remove unused `hir_id` parameter from `resolve_type`
- Fix redundant arguments in `external_path()`
- Remove unnecessary `is_trait` argument
- rustdoc: Cleanup a pattern match in `external_generic_args()`

r? ``@jyn514``

src/librustdoc/formats/cache.rs

@@ -98,9 +98,6 @@ crate struct Cache {
     stripped_mod: bool,
 
     crate search_index: Vec<IndexItem>,
-    crate deref_trait_did: Option<DefId>,
-    crate deref_mut_trait_did: Option<DefId>,
-    crate owned_box_did: Option<DefId>,
 
     // In rare case where a structure is defined in one module but implemented
     // in another, if the implementing module is parsed before defining module,