remove `rewrite_subject` action in Rspamd base config

target/rspamd/local.d/actions.conf

@@ -6,7 +6,12 @@
 # and to be able to explain the impact on the whole system.
 greylist = 4;
 add_header = 6;
-rewrite_subject = 7;
 reject = 11;
 
-subject = "***SPAM*** %s"
+# The value `null` disabled the action. A subject rewrite is handled by
+# SPAM_SUBJECT
+# (https://docker-mailserver.github.io/docker-mailserver/edge/config/environment/#spam_subject)
+#
+# The reasoning for this can be found in
+# https://github.com/docker-mailserver/docker-mailserver/issues/3804
+rewrite_subject = null;

test/tests/parallel/set1/spam_virus/rspamd_full.bats

@@ -73,6 +73,15 @@ function teardown_file() { _default_teardown ; }
   assert_output 'rspamd_milter = inet:localhost:11332'
 }
 
+@test 'Rspamd base configuration is correct' {
+  _run_in_container rspamadm configdump actions
+  assert_success
+  assert_line 'greylist = 4;'
+  assert_line 'reject = 11;'
+  assert_line 'add_header = 6;'
+  refute_line --regexp 'rewrite_subject = [0-9]+;'
+}
+
 @test "contents of '/etc/rspamd/override.d/' are copied" {
   local OVERRIDE_D='/etc/rspamd/override.d'
   _file_exists_in_container "${OVERRIDE_D}/testmodule_complicated.conf"