bufio: remove redundant private flush method from bufio package and move its code to public Flush

[nvartolomei]

https://github.com/golang/go/blob/master/src/bufio/bufio.go#L551

Looks like it is a leftover from 99f6722 and #6086.

[gopherbot]

CL https://golang.org/cl/30158 mentions this issue.