Editorial: Remove '_' RHS for IdentifierPart

... because that RHS made it ambiguous.

(Resolves issue tc39#1059.)

spec.html

@@ -9813,7 +9813,6 @@ <h2>Syntax</h2>
       IdentifierPart ::
         UnicodeIDContinue
         `$`
-        `_`
         `\` UnicodeEscapeSequence
         &lt;ZWNJ&gt;
         &lt;ZWJ&gt;
@@ -9825,6 +9824,9 @@ <h2>Syntax</h2>
         &gt; any Unicode code point with the Unicode property &ldquo;ID_Continue&rdquo;
     </emu-grammar>
     <p>The definitions of the nonterminal |UnicodeEscapeSequence| is given in <emu-xref href="#sec-literals-string-literals"></emu-xref>.</p>
+    <emu-note>
+      <p>The nonterminal |IdentifierPart| derives `_` via |UnicodeIDContinue|.</p>
+    </emu-note>
     <emu-note>
       <p>The sets of code points with Unicode properties &ldquo;ID_Start&rdquo; and &ldquo;ID_Continue&rdquo; include, respectively, the code points with Unicode properties &ldquo;Other_ID_Start&rdquo; and &ldquo;Other_ID_Continue&rdquo;.</p>
     </emu-note>