Remove the default_do_language parameter, instead making DO use a hardwired

Remove the default_do_language parameter, instead making DO use a hardwired
default of "plpgsql".  This is more reasonable than it was when the DO patch
was written, because we have since decided that plpgsql should be installed
by default.  Per discussion, having a parameter for this doesn't seem useful
enough to justify the risk of application breakage if the value is changed
unexpectedly.