remove lazy-pos forceValue

almost all uses of this are interactive, except for deepSeq. deepSeq is
going to be expensive and rare enough to not care much about, and
Value::determinePos should usually be cheap enough to not be too much of
a burden in any case.

src/libexpr/eval.hh

@@ -473,9 +473,6 @@ public:
      */
     inline void forceValue(Value & v, const PosIdx pos);
 
-    template <typename Callable>
-    inline void forceValue(Value & v, Callable getPos);
-
     /**
      * Force a value, then recursively force list elements and
      * attributes.