Tedu support for the -xsh4.2 argument to the mdoc(7) .St macro

Tedu support for the -xsh4.2 argument to the mdoc(7) .St macro
because all of the following hold:
* It is an alias for a part of an ancient standard that is no longer important.
* To refer to that old standard, -xpg4.2 is readily available and portable.
* It is unused in OpenBSD, FreeBSD, and NetBSD.
* Groff never supported it.

I agreed with G. Branden Robinson that deleting this from mandoc
is preferable to adding it to groff.