[REFACTOR] PromQL: remove label_join and label_replace stubs

These functions operate on whole series, not on samples, so they do not
fit into the table of functions that return a Vector. Remove the stub
entries that were left to help downstream users of the code identify
what changed.

We cannot remove the entries from the `FunctionCalls` map without
breaking `TestFunctionList`, so put some nils in to keep it happy.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

promql/functions.go

@@ -1514,11 +1514,6 @@ func (ev *evaluator) evalLabelReplace(ctx context.Context, args parser.Expressio
 	return matrix, ws
 }
 
-// === label_replace(Vector parser.ValueTypeVector, dst_label, replacement, src_labelname, regex parser.ValueTypeString) (Vector, Annotations) ===
-func funcLabelReplace(vals []parser.Value, args parser.Expressions, enh *EvalNodeHelper) (Vector, annotations.Annotations) {
-	panic("funcLabelReplace wrong implementation called")
-}
-
 // === Vector(s Scalar) (Vector, Annotations) ===
 func funcVector(vals []parser.Value, args parser.Expressions, enh *EvalNodeHelper) (Vector, annotations.Annotations) {
 	return append(enh.Out,
@@ -1570,11 +1565,6 @@ func (ev *evaluator) evalLabelJoin(ctx context.Context, args parser.Expressions)
 	return matrix, ws
 }
 
-// === label_join(vector model.ValVector, dest_labelname, separator, src_labelname...) (Vector, Annotations) ===
-func funcLabelJoin(vals []parser.Value, args parser.Expressions, enh *EvalNodeHelper) (Vector, annotations.Annotations) {
-	panic("funcLabelReplace wrong implementation called")
-}
-
 // Common code for date related functions.
 func dateWrapper(vals []parser.Value, enh *EvalNodeHelper, f func(time.Time) float64) Vector {
 	if len(vals) == 0 {
@@ -1696,8 +1686,8 @@ var FunctionCalls = map[string]FunctionCall{
 	"idelta":                       funcIdelta,
 	"increase":                     funcIncrease,
 	"irate":                        funcIrate,
-	"label_replace":                funcLabelReplace,
-	"label_join":                   funcLabelJoin,
+	"label_replace":                nil, // evalLabelReplace not called via this map.
+	"label_join":                   nil, // evalLabelJoin not called via this map.
 	"ln":                           funcLn,
 	"log10":                        funcLog10,
 	"log2":                         funcLog2,