Remove EE-specific code from rejection mailer

app/views/email_rejection_mailer/rejection.html.haml

@@ -2,12 +2,4 @@
   Unfortunately, your email message to GitLab could not be processed.
 
 = markdown @reason
-
--# EE-specific start
-- if Gitlab::CurrentSettings.email_additional_text.present?
-  %p
-    —
-    %br
-    %br
-      = Gitlab::Utils.nlbr(Gitlab::CurrentSettings.email_additional_text)
--# EE-specific end
+= render_if_exists 'shared/additional_email_text'

app/views/email_rejection_mailer/rejection.text.haml

 Unfortunately, your email message to GitLab could not be processed.
 \
 = @reason
-
--# EE-specific start
-- if Gitlab::CurrentSettings.email_additional_text.present?
-  \
-  = '---'
-  = Gitlab::Utils.nlbr(Gitlab::CurrentSettings.email_additional_text)
--# EE-specific end
+= render_if_exists 'shared/additional_email_text'

ee/app/views/shared/_additional_email_text.html.haml

+- return unless Gitlab::CurrentSettings.email_additional_text.present?
+
+%p
+  —
+  %br
+  %br
+    = Gitlab::Utils.nlbr(Gitlab::CurrentSettings.email_additional_text)

ee/app/views/shared/_additional_email_text.text.haml

+- return unless Gitlab::CurrentSettings.email_additional_text.present?
+\
+= '---'
+= Gitlab::Utils.nlbr(Gitlab::CurrentSettings.email_additional_text)