Remove obsolete Threading Libraries paragraph from Porters Handbook

[ak]

Summary

Preferable way is -pthread again, so remove 'Threading Libraries' paragraph from Porters Handbook

[bcr]

You used the doceng@ group, rather than the doc@ group. While that is not wrong, doc@ is the more general group of people working on documentation and your chances are higher that someone will look at your change in time.

I approve your change.

[ak]

Closed by commit rD45915 (authored by @ak).