- Remove obsolete 'Threading Libraries' paragraph

Reviewed by:	mat, bcr
Approved by:	mat (portmgr), bcr

Differential Revision:	https://reviews.freebsd.org/D997


git-svn-id: svn+ssh://svn.freebsd.org/doc/head@45915 c2e8774f-c49f-e111-b436-862b2bbc8956