kill "function ()" declaration in tracing block syntax

Since we already kill the event argument in tracing block,
so the closure argument is empty now, we can just kill the
"function ()" declaration completly.

New tracing block syntax is like:

trace "syscalls:*" {
	print(execname(), argevent)
}

trace_end {
	print("tracing end")
}

This syntax change will make one-liner be more shorter than
before.

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