libc: get rid of NO_P1003_1B make variable

There's no point in a knob to avoid installing a half dozen manpages.
It's undocumented and unused in the tree. Online, the only metions
I've found are the FreeBSD source tree, a commit in DragonFly BSD
removing it, and some lists of build options for small systems where
it's inevitably redundant due to an accompanying NO_MAN.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D33310