bitbake: doc: bitbake-user-manual: fix erroneous statement in glossary intro

This is the BitBake glossary.
Remove an erroneous statement probably coming from the introduction
to the YP/OE glossary.

(Bitbake rev: 398a1686176c695d103591089a36e25173f9fd6e)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/doc/bitbake-user-manual/bitbake-user-manual-ref-variables.rst

@@ -23,10 +23,6 @@ overview of their function and contents.
       systems extend the functionality of the variable as it is
       described here in this glossary.
 
-   -  Finally, there are variables mentioned in this glossary that do
-      not appear in the BitBake glossary. These other variables are
-      variables used in systems that use BitBake.
-
 .. glossary::
 
    :term:`ASSUME_PROVIDED`