Remove unused softhsm configuration option

slots.removable doesn't exist in older versions, andwe don't need it.
diff --git a/tests/softhsm2.conf b/tests/softhsm2.conf
index 08e94d5..a467606 100644
--- a/tests/softhsm2.conf
+++ b/tests/softhsm2.conf
@@ -1,4 +1,3 @@
 objectstore.backend=file
-slots.removable=false
 directories.tokendir=softhsm2.d
 log.level=INFO