Remove SpanInterner::get

- It's used exactly once, so it's trivial to replace
- It doesn't match the normal convention for containers: normally
`get()` returns and option and indexing panics. Instead `get()` panicked
  and there's no indexing operation available.

Author: jyn514

compiler/rustc_span/src/span_encoding.rs

@@ -102,7 +102,7 @@ impl Span {
             // Interned format.
             debug_assert!(self.ctxt_or_zero == 0);
             let index = self.base_or_index;
-            with_span_interner(|interner| *interner.get(index))
+            with_span_interner(|interner| interner.spans[index as usize])
         }
     }
 }
@@ -117,11 +117,6 @@ impl SpanInterner {
         let (index, _) = self.spans.insert_full(*span_data);
         index as u32
     }
-
-    #[inline]
-    fn get(&self, index: u32) -> &SpanData {
-        &self.spans[index as usize]
-    }
 }
 
 // If an interner exists, return it. Otherwise, prepare a fresh one.