erlang: install manpages only in lib/erlang/man

Some erlang man pages have generic names that conflict with other
packages. The recommended installation location is lib/erlang/man, and
in fact they must be there for the "erl -man" command to find them.
Currently we use symlinks to make this work.

Instead let's only install them to lib/erlang/man and avoid all
conflicts. Users can add $(brew --prefix erlang)/lib/erlang/man to
$MANPATH if they really want to use man to find them.