ref-manual: drop a sentence about a 'legacy way' to specify UNPACKDIR

This wasn't specific, and is obsolete: UNPACKDIR should not be
set by recipes at all, except in very special circumstances
(e.g. gcc/clang unpacking into work-shared).

(From yocto-docs rev: a880d94d70c7ed1da8e40a02dae43ff1699941a6)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/tasks.rst

@@ -412,8 +412,7 @@ them. You can learn more by looking at the
 -------------
 
 Unpacks the source code into a working directory pointed to by
-``${``\ :term:`UNPACKDIR`\ ``}``. A legacy way to specify
-this directory is through the :term:`S` and :term:`WORKDIR` variables.
+``${``\ :term:`UNPACKDIR`\ ``}``.
 For more information on how source files are unpacked, see the
 ":ref:`overview-manual/concepts:source fetching`"
 section in the Yocto Project Overview and Concepts Manual.