Remove support for backticks from the grammar and compiler.

Still need to remove traces of the UNARY_CONVERT opcode.
diff --git a/Python/ast.c b/Python/ast.c
index f472d96..b566ba3 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -1190,7 +1190,7 @@
 ast_for_atom(struct compiling *c, const node *n)
 {
     /* atom: '(' [yield_expr|testlist_gexp] ')' | '[' [listmaker] ']'
-       | '{' [dictmaker] '}' | '`' testlist '`' | NAME | NUMBER | STRING+
+       | '{' [dictmaker] '}' | NAME | NUMBER | STRING+
     */
     node *ch = CHILD(n, 0);
     
@@ -1276,13 +1276,6 @@
 	}
 	return Dict(keys, values, LINENO(n), n->n_col_offset, c->c_arena);
     }
-    case BACKQUOTE: { /* repr */
-	expr_ty expression = ast_for_testlist(c, CHILD(n, 1));
-	if (!expression)
-	    return NULL;
-
-	return Repr(expression, LINENO(n), n->n_col_offset, c->c_arena);
-    }
     default:
 	PyErr_Format(PyExc_SystemError, "unhandled atom %d", TYPE(ch));
 	return NULL;