libc: get rid of NO_P1003_1B make variable

[brooks]

Summary

There's no point in a knob to avoid installed a half dozen manpages.
It's undocumented and unused in the tree. Online, the only metions
I've found are to 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.

[emaste]

NO_P1003_1B dates to R10:8a6472b7239ccfab9d1ce96671a9837813698499. I guess it was just to simplify introduction when this was WIP.