Revert ""constructor" is now a terminal, so prohibiting it as an identifier is redundant"

Author: bathos

index.bs

@@ -715,7 +715,8 @@ keyword token is used, then the [=identifier=] of the operation argument
 is simply that token.
 
 The [=identifier=] of any of the abovementioned
-IDL constructs (except operation arguments) must not be "<code>toString</code>",
+IDL constructs (except operation arguments) must not be "<code>constructor</code>",
+"<code>toString</code>",
 or begin with a <span class="char">U+005F LOW LINE ("_")</span> character.  These
 are known as <dfn id="dfn-reserved-identifier" export>reserved identifiers</dfn>.