Remove Makefile targets and tools related to Irix and Tru64 debugging tools.

Remove Makefile targets and tools related to Irix and Tru64 debugging tools.

Remove the targets:

perl.pixie perl.pixie.atom perl.pixie.config perl.pixie.irix perl.third
perl.third.config

It's still possible to run the actions these targets "by hand", if desired.
This commit removes the convenience targets from the Makefile, reducing its
complexity. It also removes the related support scripts testall.atom and
thirdclean from Porting/

pixie is a performance analysis tool for Irix and Tru64
Third Degree is a memory checker tool for Tru64

Given that Tru64 went out of support at the end of 2012, and Irix goes out
of support at the end of 2013, it's very unlikely that anyone is still
actively profiling or debugging perl on either platform, and hence using
these targets. It's been several years since we've even had a regular bug
report from either platform.