libsys manuals: Move library to SYNOPSIS

[ziaee]

Summary

The LIBRARY section is deprecated, and upstream is asking us to move
Lb to SYNOPSIS. We've already done this in a few manuals as we've been
working on them, but I finally figured out how to do it mechanically!
I used:

for i in lib/libsys/*.2
do ed $i <<EOE
/.Lb/
m+
/.Sh LIBRARY/d
wq
EOE
done

Thanks: mwl (I learned this from Ed Mastery, which was a very fun read!)