change trace block syntax: remove eventdef string quotation marks

This will make trace block syntax be more simpler, and
easy to write one-liner in command line.

Example:

Before patch:
	trace "syscalls:*" {
		print(argevent)
	}

After patch:
	trace syscalls:* {
		print(argevent)
	}

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>