Use new YS 'eval' std fn in 'strain'

exercises/practice/strain/.meta/strain.ys

@@ -5,11 +5,3 @@ defn keep(list predicate):
 
 defn discard(list predicate):
   remove eval(predicate): list
-
-# XXX: eval targeted to be in ys::std on 0.1.80
-defn eval(code):
-  str("!yamlscript/v0\n$code"):
-    .ys/compile()
-    .replace(/(?s)^\(\+\+\+ (.*)\)$/ '$1')
-    .read-string()
-    .clojure::core/eval()