migration-guides: QEMU_USE_SLIRP variable removed

(From yocto-docs rev: f50e9fe501ccafd18ed2d8a9e505be503a721846)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/migration-guides/migration-4.3.rst

@@ -129,6 +129,7 @@ The following variables have been removed:
 -  ``AUTHOR``
 -  ``PERLARCH``
 -  ``PERLVERSION``
+-  ``QEMU_USE_SLIRP``
 
 .. _migration-4.3-removed-recipes: