Remove vestigial comment in advancedSymbols.js

TeX backslash control sequences are letters-only, whereas HTML character
entity references are alphanumeric; therefore `∴` is a valid
character entity reference, but `\there4` cannot be a valid control
sequence. Initially I hoped to support every character entity reference
as a backslash command, but we can't support this one, so forget it.

The out-of-place "case" is because backslash commands used to be a
switch/case, not a dictionary:
mathquill@77bd6c4#diff-239c238d8c090904fa1a9261bf2ef674L961

Author: laughinghan

src/commands/math/advancedSymbols.js

@@ -16,7 +16,7 @@ LatexCmds['≠'] = LatexCmds.ne = LatexCmds.neq = bind(BinaryOperator,'\\ne ','&
 
 LatexCmds['∗'] = LatexCmds.ast = LatexCmds.star = LatexCmds.loast = LatexCmds.lowast =
   bind(BinaryOperator,'\\ast ','∗');
-  //case 'there4 = // a special exception for this one, perhaps?
+
 LatexCmds.therefor = LatexCmds.therefore =
   bind(BinaryOperator,'\\therefore ','∴');