Remove the unnecessary helpline function and help_line variable

Despite the subSilver2 abandonment since phpBB 3.2.0, the helpline function is still present in editor.js and the help_line variable is still defined in the template files posting_buttons.html and acp_posting_buttons.html.

But these JavaScript function and variable are useless in proSilver, probably in any prosilver-based style too, and they should be removed to clean the code. Unlike subSilver2, that helpline function is not called when cursor is on BBcode buttons or out (onmouseover and onmouseout Javascript events). Instead, proSilver uses the title attribute for each BBcode button to show the helpline.

Deprecated old subSilver2, in phpBB 3.1 :

<input type="button" class="button2" accesskey="b" name="addbbcode0" value="B" style="font-weight: bold; width: 30px" onclick="bbstyle(0)" onmouseover="helpline('b')" onmouseout="helpline('tip')" />

Newer proSilver, in phpBB 3.2 :

<button type="button" class="button button-icon-only bbcode-b" accesskey="b" name="addbbcode0" value=" B " onclick="bbstyle(0)" title="{L_BBCODE_B_HELP}">

<i class="icon fa-bold fa-fw" aria-hidden="true"></i>

</button>

onmouseover="helpline('b')" was replaced by title="{L_BBCODE_B_HELP}", and JavaScript helpline function and help_line variable are no longer used.