Remove the explicit purify/quantify/purecov targets and documentation.

Remove the explicit purify/quantify/purecov targets and documentation.

It's not clear whether IBM still sell quantify or purecov. They still seem
to sell purify, but I'm not sure if anyone is using it these days to detect
bugs in perl.

This doesn't prevent anyone from using these tools if they have them, as
it's still possible to run the commands by "hand". But by removing probably
unused code and documentation, the signal to noise ratio improves.