getting rid of maintainer-check

The maintainer-check target never really caught on, I think.  Most
people don't run it, and that in turn annoys those who do.  Also, it
doesn't provide much functionality.

I propose that we get rid of it and roll the functionality into the
regular build.

Specifically:

- Running duplicate_oids during the regular build was already discussed
elsewhere recently.  There are some details to be resolved there, but
it's doable.

- Checking for tabs in SGML files can be run during the regular
documentation build without problems.

- The NLS checks can also be run during the regular NLS-enabled build.

That's it.  Any concerns?