Remove res.flush

[okkedek]

Removes res.flush(), causes error on express 4.17 (TypeError: res.flush is not a function) and occurs on res.write anyway.

[t3hmrman]

A decent workaround is to just add a res.flush that express-sse can call that and have it do nothing, roughly:

res.flush = () => undefined;

[x1unix]

@okkedek @t3hmrman @dpskvn res.flush() is deprecated. Use res.flushHeaders() instead.

- res.flush()
+ (res.flushHeaders || res.flush )()