MFC: Remove confDH_PARAMETERS settings in favor of using sendmail's
built-in default which was added in sendmail 8.15.2 (the config
line predates that 8.15.2 feature). This also alleviates the need
for admins to create the DH parameters file if they opt to use
Diffie-Hellman.

PR: 248387

(cherry picked from commit 98fd1add676321978db72d77d34ef51ca454c814)