Remove replace directives and use own fork to make go get work again (#1028)

See https://github.com/golang/go/issues/30354
go get doesn't honor the go.mod replace options.

vendor/github.com/bwmarrin/discordgo/util.go

@@ -1,17 +0,0 @@
-package discordgo
-
-import (
-	"strconv"
-	"time"
-)
-
-// SnowflakeTimestamp returns the creation time of a Snowflake ID relative to the creation of Discord.
-func SnowflakeTimestamp(ID string) (t time.Time, err error) {
-	i, err := strconv.ParseInt(ID, 10, 64)
-	if err != nil {
-		return
-	}
-	timestamp := (i >> 22) + 1420070400000
-	t = time.Unix(timestamp/1000, 0)
-	return
-}