Don't add an ID to every pie label

This causes issues on pages with more than one pie chart, as duplicate label IDs are created, which can cause issues with accessibility (http://www.w3.org/TR/WCAG20-TECHS/F77).

Additionally, the use case for having an ID on each pie chart label seems to be negligible / non-existent, and it appeared to be simply a means of selecting the label further down in the code which is not necessary.