Disable new smooth updates

Setting SMOOTH_UPDATES to a string which is neither a valid section
name (nor the magic string "ALL") should allow removal of old libraries
to continue without any new smooth updated libraries being accepted.

An empty SMOOTH_UPDATES would also stop removals from occurring.

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>

britney.conf

@@ -53,4 +53,7 @@ HINTS_SATBRITNEY  = easy
 
 # support for old libraries in testing (smooth update)
 # use ALL to enable smooth updates for all the sections
-SMOOTH_UPDATES    = libs oldlibs
+#
+# naming a non-existent section will effectively disable new smooth
+# updates but still allow removals to occur
+SMOOTH_UPDATES    = badgers