Remove unneeded attribute

This was part of the initial commit (in 2014), but does not appear
to have been used for anything at a later stage.  So good riddance!

Author: lizmat

lib/OO/Monitors.rakumod

@@ -1,6 +1,5 @@
 class MetamodelX::MonitorHOW is Metamodel::ClassHOW {
     has $!lock-attr;
-    has %!condition-attrs;
 
     method new_type(|) {
         my \type = callsame();