reader handle_completions(): remove dead code for skipping to add prefix

The tuple (will_replace_token, all_matches_exact_or_prefix) can never
be (false, false).

Proof by contraction:
1. Goal: show unsatisfiability of: !will_replace_token && !all_matches_exact_or_prefix
2. Substitute defintions: !all(replaces) && !all(is_exact_or_prefix)
3. wlog, !replaces(c1) && !is_exact_or_prefix(c2)
4. since c1 and c2 have same rank we know that !is_exact_or_prefix(c1)
5. !is_exact_or_prefix() implies requires_full_replacement()
6. all callers that create a Completion from StringFuzzyMatch::try_create(),
   set CompleteFlags::REPLACE_TOKEN if requires_full_replacement(),
   so requires_full_replacement() implies replaces()
7. From 4-6 follows: !is_exact_or_prefix(c1) implies replaces(c1), which is a contradiction

src/reader/reader.rs

@@ -6808,7 +6808,8 @@ fn handle_completions(&mut self, token_range: Range<usize>, comp: Vec<Completion
 
         // Print the completion list.
         let mut prefix = WString::new();
-        if will_replace_token || !all_matches_exact_or_prefix {
+        assert!(will_replace_token || all_matches_exact_or_prefix);
+        if will_replace_token {
             if use_prefix {
                 prefix.push_utfstr(&common_prefix);
             }