Remove unused cache_lookaside_score and cache_hs_score stats

The cache_lookaside_score and cache_hs_score statistics are no longer used by MongoDB.  They don't seem to serve any useful purpose in the WT code either. cache_lookaside_score in particular is simply set to zero and never changed (see WT-5576).  

WT-5499 discussed finding a new statistics to replace cache_lookaside_score when we replaced the lookaside file with durable history. But that ticket was never closed and according to dianna.hohensee we have not used cache_lookaside_score since v4.4.

Unless these statistics are somehow needed for backwards compatibility, we should remove them.