Remove unused single_step flag

This appears to have never been used ever since its introduction in 61c7569 --
the plugin discussed on the PR introducing that commit, 34811, never
materialized.

It's also simple to readd in the current scheme, but given that macro expansion
is already quite complicated, additional useless state seems good to remove
while we're not using it.

Author: Mark-Simulacrum

src/librustc_expand/expand.rs

@@ -507,9 +507,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                 expanded_fragments.push(Vec::new());
             }
             expanded_fragments[depth - 1].push((expn_id, expanded_fragment));
-            if !self.cx.ecfg.single_step {
-                invocations.extend(new_invocations.into_iter().rev());
-            }
+            invocations.extend(new_invocations.into_iter().rev());
         }
 
         self.cx.current_expansion = orig_expansion_data;
@@ -1819,7 +1817,6 @@ pub struct ExpansionConfig<'feat> {
     pub recursion_limit: usize,
     pub trace_mac: bool,
     pub should_test: bool, // If false, strip `#[test]` nodes
-    pub single_step: bool,
     pub keep_macs: bool,
 }
 
@@ -1831,7 +1828,6 @@ impl<'feat> ExpansionConfig<'feat> {
             recursion_limit: 1024,
             trace_mac: false,
             should_test: false,
-            single_step: false,
             keep_macs: false,
         }
     }