Add overload to remove specific class of buff

I initially added this because, despite the higher tickrate for
composite condition calculations thanks to the last commit, there is
still a slight status icon delay when rapidly switching nanos. I
attempted to use this to make that problem go away and for whatever
reason it wasn't effective, but I figure it would be useful to have
anyway so I'm keeping it.

src/Buffs.hpp

@@ -54,6 +54,7 @@ public:
     void tick(time_t);
     void combatTick(time_t);
     void clear();
+    void clear(BuffClass buffClass);
     void addStack(BuffStack* stack);
 
     /*