Clarify uniqueness of F17

Currently uniqueness or necessity of F17 is unclear. F17 discusses

- uniqueness of id attribute
- correctness of referencing id or unique identifies (for, headers, usemap attributes)
- uniqueness of accesskey attribute

However F77 and F62 discusses most of them.

- F77 discusses uniqueness of id
- F62 discusses correctness of referencing id or unique identifiers
    - F68 discusses that in for attribute
    - F90 discusses that in headers attribute

Remaining issue is uniqueness of accesskey attribute. So we should remove other issuse from F17.

- [F17: Failure of Success Criterion 1.3.1 and 4.1.1 due to insufficient information in DOM to determine one-to-one relationships (e.g., between labels with same id) in HTML](http://www.w3.org/WAI/GL/2014/WD-WCAG20-TECHS-20140724/F17.html)
- [F62: Failure of Success Criterion 1.3.1 and 4.1.1 due to insufficient information in DOM to determine specific relationships in XML](http://www.w3.org/WAI/GL/2014/WD-WCAG20-TECHS-20140724/F62.html)
- [F68: Failure of Success Criterion 1.3.1 and 4.1.2 due to the association of label and user interface controls not being programmatically determined ](http://www.w3.org/WAI/GL/2014/WD-WCAG20-TECHS-20140724/F68.html)
- [F77: Failure of Success Criterion 4.1.1 due to duplicate values of type ID](http://www.w3.org/WAI/GL/2014/WD-WCAG20-TECHS-20140724/F77.html)
- [F90: Failure of Success Criterion 1.3.1 for incorrectly associating table headers and content via the headers and id attributes](http://www.w3.org/WAI/GL/2014/WD-WCAG20-TECHS-20140724/F90.html)