syntax: Remove SyntaxExtension::DeclMacro

It's a less powerful duplicate of `SyntaxExtension::NormalTT`

src/test/ui/macros/nonterminal-matching.stderr

@@ -1,6 +1,9 @@
 error: no rules expected the token `enum E { }`
   --> $DIR/nonterminal-matching.rs:19:10
    |
+LL |     macro n(a $nt_item b) {
+   |     --------------------- when calling this macro
+...
 LL |     n!(a $nt_item b);
    |          ^^^^^^^^ no rules expected this token in macro call
 ...