dev-db/postgresql: 10beta2 Remove dodoc *{sgml,dsl}

dev-db/postgresql: 10beta2 Remove dodoc *{sgml,dsl}

When USE="doc" is enabled, ‘dodoc doc/src/sgml/*.{sgml,dsl}’ fails
because the *.dsl files have been removed.

The SGML files are just the source code used to generate the HTML
files that are installed a couple lines earlier, and are, for the most
part, a waste of space.

So, we’re going to stop installing them.

Bug: 624914

Package-Manager: Portage-2.3.6, Repoman-2.3.1