Remove the Postfix plugin (#7097)

* Remove the postfix plugin.

* Remove references to postfix plugin in code.

* Remove reference to postfix plugin in docs.

tools/_venv_common.py

@@ -39,7 +39,6 @@ REQUIREMENTS = [
     '-e certbot-dns-route53',
     '-e certbot-dns-sakuracloud',
     '-e certbot-nginx',
-    '-e certbot-postfix',
     '-e letshelp-certbot',
     '-e certbot-compatibility-test',
     '-e certbot-ci',

tools/install_and_test.py

@@ -14,8 +14,7 @@ import shutil
 import subprocess
 import re
 
-SKIP_PROJECTS_ON_WINDOWS = [
-    'certbot-apache', 'certbot-postfix', 'letshelp-certbot']
+SKIP_PROJECTS_ON_WINDOWS = ['certbot-apache', 'letshelp-certbot']
 
 
 def call_with_print(command, cwd=None):