Remove default config for smtpd_tls_cert_file and smtpd_tls_key_file to avoid warning message in logs

[guipe]

When TLS is not used, if we let default configuration:

smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key

These warning messages appears in /var/log/maillog:

Jul 29 14:24:01 postfix/smtpd[24123]: warning: cannot get RSA certificate from file "/etc/ssl/certs/ssl-cert-snakeoil.pem": disabling TLS support
Jul 29 14:24:01 postfix/smtpd[24123]: warning: TLS library problem: error:02001002:system library:fopen:No such file or directory:../crypto/bio/bss_file.c:288:fopen('/etc/ssl/certs/ssl-cert-snakeoil.pem','r'):
Jul 29 14:24:01 postfix/smtpd[24123]: warning: TLS library problem: error:20074002:BIO routines:file_ctrl:system lib:../crypto/bio/bss_file.c:290:
Jul 29 14:24:01 postfix/smtpd[24123]: warning: TLS library problem: error:140DC002:SSL routines:use_certificate_chain_file:system lib:../ssl/ssl_rsa.c:596:

As these files are removed from Dockerfile command, we must remove these ones from main.cf default configuration.

[mastamark]

As a note I too was trying to run with USE_TLS=no and I was additionally having some issues with clients connecting and running into the default values in main.cf for these 2:
smtpd_tls_security_level=may
smtp_tls_security_level=may

Setting them both to none solved the problem for me as found from here.

The provided params to use to tweak these settings (TLS_SECURITY_LEVEL && CLIENT_TLS_SECURITY_LEVEL) were not being consumed or set because of the overriding USE_TLS=no.

I am able to get around it by a few different ways - another set of seds in the dockerfile or an else condition on the USE_TLS if loop for when its no to update, or some other option I didn't think of but I figured I'd call this out for anyone else who was trying to fully disable TLS and running into problems still even after using the above fix.