Remove id attributes from JS documentation

[wbamberg]

This is based on #3607, "[Markdown] id attributes in the JavaScript documentation", and removes all id attributes from our JS documentation, except those attached to <h1>...<h6> elements.

Or at least I think it does.

What I did:

• grepped files/en-us/web/javascript for id="
• excluded those attached to <h1>...<h6>, and those that just appeared in sample code

This gave me:

./files/en-us/web/javascript//guide/grammar_and_types/index.html: <li id="parseInt()_and_parseFloat()">{{jsxref("parseInt", "parseInt()")}}</li>
./files/en-us/web/javascript//guide/regular_expressions/index.html:<p id="g-different-behaviors">The behavior associated with the <code>g</code> flag is different when the <code>.exec()</code> method is used.  The roles of "class" and "argument" get reversed: In the case of <code>.match()</code>, the string class (or data type) owns the method and the regular expression is just an argument, while in the case of <code>.exec()</code>, it is the regular expression that owns the method, with the string being the argument.  Contrast this <em><code>str.match(re)</code></em> versus <em><code>re.exec(str)</code></em>.  The <code>g</code> flag is used with the <strong><code>.exec()</code></strong> method to get iterative progression.</p>
./files/en-us/web/javascript//guide/details_of_the_object_model/index.html:<p><img alt="" class="internal" id="figure8.5" src="figure8.5.png"><br>
./files/en-us/web/javascript//inheritance_and_the_prototype_chain/index.html:<div class="notecard warning" id="Bad_practice_Extension_of_native_prototypes">
./files/en-us/web/javascript//inheritance_and_the_prototype_chain/index.html:<pre id="example-4" class="brush: js">
./files/en-us/web/javascript//reference/operators/in/index.html:      id="result_box" lang="en">contains</span> the property with specified name
./files/en-us/web/javascript//reference/operators/inequality/index.html:<div id="gtx-trans" style="position: absolute; left: 16px; top: 1743.2px;">
./files/en-us/web/javascript//reference/statements/let/index.html:<p id="Scoping_rules">At the top level of programs and functions,
./files/en-us/web/javascript//reference/global_objects/regexp/exec/index.html:      <td><code id="indices">indices</code></td>
./files/en-us/web/javascript//reference/global_objects/atomics/index.html:<div id="gtx-trans" style="position: absolute; left: 25px; top: 2197px;">
./files/en-us/web/javascript//reference/global_objects/string/split/index.html:<div id="split_empty_string">
./files/en-us/web/javascript//reference/strict_mode/index.html:      id="sloppyModeId333">sloppy mode</a>"</strong>. This isn't an official term, but be

(this omits some items from the list in #3607 - on closer inspection, several of those are actually attached to headings, and one has since been removed by another PR.)

I grepped the entire files/en-us tree for references to each id attribute:

• Where there were no references I just removed the id attribute.
• Where there was a reference I replaced it with a reference to a heading ID. The only place I wasn't able to do that was in the Firefox 88 release notes, which wants to refer to #indices, and it would be hard to reorganize that page so indices gets a heading. But I think it's OK to just link to the page here.

[chrisdavidmills]

This looks good to me, thanks @wbamberg !