cmd/compile: remove ir.OSTMTEXPR op

It is not used anywhere.

Change-Id: I2b8d64902e16893ab5e1b84943f6d865c6e9c7bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/311950
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

Author: cuonglm

src/cmd/compile/internal/ir/node.go

@@ -248,7 +248,6 @@ const (
 	OOFFSETOF    // unsafe.Offsetof(X)
 	OSIZEOF      // unsafe.Sizeof(X)
 	OMETHEXPR    // method expression
-	OSTMTEXPR    // statement expression (Init; X)
 
 	// statements
 	OBLOCK // { List } (block of code)