include: Remove symlink installation

headers could be installed as symlink to the source tree instead of copies.
Remove the possibility to do that.
This make the makefile easier to read and to maintain and also don't duplicate
code.

While here remove some directories from LSBUDIRS as we already install them using
the INCS stuff.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D29167
MFC after: 2 weeks