Removed TODO.

Tested whether we needed to account for \ escapes in backtick
literals in mysql REPL:
> CREATE DATABASE scratch;
> USE SCRATCH;
> CREATE TABLE `a``b` (id INT);
> SELECT * FROM `a``b`;
Empty set (0.01 sec)
> SELECT * FROM `a\`b`;
ERROR 1064 (42000): You have an error in your SQL syntax; ...

and by examination of https://dev.mysql.com/doc/refman/5.7/en/string-literals.html
re other escaping conventions.

lib/SqlString.js

@@ -14,7 +14,8 @@ var CHARS_ESCAPE_MAP    = {
   '\''   : '\\\'',
   '\\'   : '\\\\'
 };
-var ONE_ID_PATTERN = '`(?:[^`]|``)+`'; // TODO(mikesamuel): Should allow ``?
+// \ does not escape backquotes inside ID literals.
+var ONE_ID_PATTERN = '`(?:[^`]|``)+`';
 // One or more Identifiers separated by dots.
 var QUALIFIED_ID_REGEXP = new RegExp(
   '^' + ONE_ID_PATTERN + '(?:[.]' + ONE_ID_PATTERN + ')*$');