[css-syntax-3] Remove the now-redundant early check for a valid property start.

Author: tabatkins

css-syntax-3/Overview.bs

@@ -2336,10 +2336,6 @@ Parse a declaration</h4>
 			<li>
 				[=token stream/Discard whitespace=] from |input|.
 
-			<li>
-				If the <a>next token</a> from |input| is not an <<ident-token>>,
-				return a syntax error.
-
 			<li>
 				<a>Consume a declaration</a> from |input|.
 				If anything was returned, return it.