common-licenses/OASIS: remove unused license

This isn't a common license as it's the DocBook v4 XML DTD license, and
that is now part of the recipe itself.

(From OE-Core rev: fd0384d285d010207f23fc55fe4dae27cea0030e)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/files/common-licenses/OASIS

-  Permission to use, copy, modify and distribute the DocBook DTD and
-  its accompanying documentation for any purpose and without fee is
-  hereby granted in perpetuity, provided that the above copyright
-  notice and this paragraph appear in all copies.  The copyright
-  holders make no representation about the suitability of the DTD for
-  any purpose.  It is provided "as is" without expressed or implied
-  warranty.
-
-  If you modify the DocBook DTD in any way, except for declaring and
-  referencing additional sets of general entities and declaring
-  additional notations, label your DTD as a variant of DocBook.  See
-  the maintenance documentation for more information.
-