Drop "Use Keep-Alive" option altogether

**Michael Osipov** ([Bug 62262](https://bz.apache.org/bugzilla//show_bug.cgi?id=62262&redirect=false)):
JMeter 4.0 sports HttpClient 4.5.x and Java URLConnection by default. Both use HTTP/1.1 only, one cannot control this in JMeter anyway (for the good). Having the option "Use Keep-Alive" makes no sense. HTTP/1.1 is persistent by default. Infact, "Connection: Keep-Alive" is an invalid header field for HTTP/1.1. Closing is a performance penalty for client and server which is the default in a new test plan.

I see no benefit to have this option around at all anymore. For those who want to fiddle with connections can use a header manager for "Connection: close".

The enhancement request to remove this altogether from GUI and code.

OS: All