testimage.bbclass: remove QEMU_USE_SLIRP variable

QEMU_USE_SLIRP is replaced by TEST_RUNQEMUPARAMS with "slirp" and
possibly other arguments to runqemu script.

(From OE-Core rev: 99fd24f0d9ff79fed389ae5a01c3031d7e7167d0)

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-recipe/testimage.bbclass

@@ -322,7 +322,7 @@ def testimage_main(d):
     ovmf = d.getVar("QEMU_USE_OVMF")
 
     slirp = False
-    if d.getVar("QEMU_USE_SLIRP") or bb.utils.contains('TEST_RUNQEMUPARAMS', 'slirp', True, False, d):
+    if bb.utils.contains('TEST_RUNQEMUPARAMS', 'slirp', True, False, d):
         slirp = True
 
     # TODO: We use the current implementation of qemu runner because of