lib/logstorage: removed misleading comment from the Query.dropPipesUnsafeForHits()

There is no need in mentioning a special case for union() pipe, for join() pipe
and any other pipe exceptions, which are going to be added in the future,
since this is superflouos information, which can become outdated or misleading over time.

This is a follow-up for fb6dafb

Updates #641

Author: valyala

lib/logstorage/parser.go

@@ -543,8 +543,6 @@ func (q *Query) AddCountByTimePipe(step, off int64, fields []string) {
 
 // dropPipesUnsafeForHits drops trailing pipes from q, which are unsafe
 // for calculating hits grouped by _time.
-//
-// It preserves union() pipes if they do not modify _time.
 func (q *Query) dropPipesUnsafeForHits() {
 	for i, p := range q.pipes {
 		if !isPipeSafeForHits(p) {