http: remove redundant condition

conn.destroyed is guaranteed to be false because a previous if statement already handles the case where conn && conn.destroyed evaluates to true returning false in that case.

https://github.com/nodejs/node/blob/83495e778319603c3ed61e31d37dc9715df14329/lib/_http_outgoing.js#L259-L263

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines