tls: remove Curl_ssl false_start

tls: remove Curl_ssl false_start

The secure transport vTLS backend was the only Curl_ssl struct instance
that populated the false_start field. Since its removed, we can now
remove that field entirely. This was a protocol feature specific to TLS
1.2 that has been replaced by the more widely adopted TLS 1.3 early data
mechanisms.

--false-start is now deprecated

Closes #17595