runtime: unwire g/m in dropg always

Currently dropg does not unwire locked g/m.
This is unnecessary distiction between locked and non-locked g/m.
We always restart goroutines with execute which re-wires g/m.

First, this produces false sense that this distinction is necessary.
Second, it can confuse some sanity and cross checks. For example,
if we check that g/m are unwired before we wire them in execute,
the check will fail for locked g/m. I've hit this while doing some
race detector changes, When we deschedule a goroutine and run
scheduler code, m.curg is generally nil, but not for locked ms.

Remove the distinction.

Change-Id: I3b87a28ff343baa1d564aab1f821b582a84dee07
Reviewed-on: https://go-review.googlesource.com/19950Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/proc.go

@@ -2081,10 +2081,8 @@ top:
 func dropg() {
 	_g_ := getg()
 
-	if _g_.m.lockedg == nil {
 	_g_.m.curg.m = nil
 	_g_.m.curg = nil
-	}
 }
 
 func parkunlock_c(gp *g, lock unsafe.Pointer) bool {