Retiring pg_regress' --load-language option

pg_regress' --load-language option has been unused in the core code since
we introduced extensions in 9.1; all callers now use --load-extension
instead. It's a fairly safe bet that it's never been used by any non-core
code, since it could only work for languages listed in pg_pltemplate.
The last possible reason to use it expired with commit 50fc694e4, which
removed pg_pltemplate and made parameterless CREATE LANGUAGE equivalent
to CREATE EXTENSION. So I think we might as well kill it, as per the
attached trivial patch. Any objections?

regards, tom lane