YAGNI active_resolve_capabilities

This leaks a lot of LSP details into ide layer, which we want to avoid:

c9cec381bc/docs/dev (lsp-independence)

Additionally, all what this infra does is providing a toggle for
auto-import completion, but we already have one!

crates/completion/src/lib.rs

@@ -20,7 +20,7 @@ use text_edit::TextEdit;
 use crate::{completions::Completions, context::CompletionContext, item::CompletionKind};
 
 pub use crate::{
-    config::{CompletionConfig, CompletionResolveCapability},
+    config::CompletionConfig,
     item::{CompletionItem, CompletionItemKind, CompletionScore, ImportEdit, InsertTextFormat},
 };