Delete support for FFS filesystems before the in-inode symlink

Delete support for FFS filesystems before the in-inode symlink
optimization.  As observed by ali_farzanrad(at)riseup.net, support
for these was broken in the 5.5 release in early 2014 by the time_t
changes.  No one noticed before now, so clearly this isn't something
we need to continue to support; rejecting in ffs_validate() is an
improvement.

Also: simplify DIRSIZ(), drop OLDDIRFMT and NEWDIRFMT, tests of
fs_maxsymlinklen against zero, #ifdef tests of FS_44INODEFMT, and
remove support for newfs -O0, last used in 2016.

ok miod@