Revert "provide a better panic message when no jobs in scheduler and start blocking called"

[JohnRoesler]

Reverts #501

There are cases where it can be called and not cause a deadlock, so I'm just going to leave it alone for now.

example:

package main

import (
	"log"
	"time"

	"github.com/go-co-op/gocron"
)

func main() {
	go func() {
		for {
			time.Sleep(time.Second)
			log.Println("doing something")
		}
	}()

	s := gocron.NewScheduler(time.UTC)
	s.StartBlocking()
}