tcp: Remove a redundant net_epoch entry in tcp6_connect()

tcp6_connect() is always called in a net_epoch read section.

Fixes: 3d76be28ec60 ("netinet6: require network epoch for in6_pcbconnect()")
Reviewed by: tuexen, glebius
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D38506