Remove useless Asserts in Result Cache code

Testing if an unsigned variable is >= 0 is pretty pointless.

There's likely enough code in remove_cache_entry() to verify the cache
memory accounting is correct in assert enabled builds. These Asserts
were not adding much extra cover, even if they had been checking >= 0 on a
signed variable.

Reported-by: Andres Freund
Discussion: https://postgr.es/m/20210402204734.6mo3nfacnljlicgn@alap3.anarazel.de

src/backend/executor/nodeResultCache.c

@@ -286,8 +286,6 @@ entry_purge_tuples(ResultCacheState *rcstate, ResultCacheEntry *entry)
 
 	/* Update the memory accounting */
 	rcstate->mem_used -= freed_mem;
-
-	Assert(rcstate->mem_used >= 0);
 }
 
 /*
@@ -346,8 +344,6 @@ remove_cache_entry(ResultCacheState *rcstate, ResultCacheEntry *entry)
 	 */
 	rcstate->mem_used -= EMPTY_ENTRY_MEMORY_BYTES(entry);
 
-	Assert(rcstate->mem_used >= 0);
-
 	/* Remove the entry from the cache */
 	resultcache_delete_item(rcstate->hashtable, entry);