[doc] remove obsolete reference

This got glossed over when the code of ``luaotfload.lua`` was ripped out
of the DTX. No links into the implementation, for the time being.
[build] remove obsolete targets

The target ``all`` didn’t work for quite a long time and ``world`` was
more or less an alias for it. The ``ctan`` target does everything now,
with ``all`` as an alias.