MDL-78198 file: Stop sending the Accept-Charset header in curl

See notes on:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Charset

Warning: Do not use this header. Browsers omit this header and servers
should ignore it.

The Accept-Charset request HTTP header was a header that advertised a
client's supported character encodings. It is no longer widely used.

UTF-8 is well-supported and the overwhelmingly preferred choice for
character encoding. To guarantee better privacy through less
configuration-based entropy, all browsers omit the Accept-Charset
header.

lib/filelib.php

@@ -3513,7 +3513,6 @@ class curl {
         if (empty($this->header)) {
             $this->setHeader(array(
                 'User-Agent: ' . $useragent,
-                'Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7',
                 'Connection: keep-alive'
                 ));
         } else if (!in_array('User-Agent: ' . $useragent, $this->header)) {