Revert "minor: Support html content in mailer.php"

This reverts commit b91ffae2.

classes/mailer.php

@@ -45,15 +45,7 @@ class Mailer {
 			++$hooks_tried;
 		}
 
-		$headers = [ "From: $from_combined" ];
-
-		if($message_html){
-			$headers[]="MIME-Version: 1.0";
-			$headers[]="Content-Type: text/html; charset=UTF-8";
-		}
-		else{
-			$headers[]="Content-Type: text/plain; charset=UTF-8";
-		}
+		$headers = [ "From: $from_combined", "Content-Type: text/plain; charset=UTF-8" ];
 
 		$rc = mail($to_combined, $subject, $message, implode("\r\n", array_merge($headers, $additional_headers)));