Remove deprecated argument from wp_get_http_headers

[borgesbruno]

There is a deprecated argument in wp_get_http_headers since the 2.7 WordPress version that isn't used in any place, actually there is only one call for this function in src/wp-includes/functions.php if ( $headers = wp_get_http_headers( $url) ) {, which do not use this argument, so, seems there is no need for this argument, in aditional I've wrote tests to cover this function and improve the coverage.

If there is some utility for this let me know, because the unit tests will be useful

[rmccue]

This argument is still on the function for two reasons: a) so that people who were using it get a deprecation warning, and b) so that we don't accidentally introduce a conflicting argument in future changes.

The unit tests might be useful though; can you submit that as a new ticket please?

[borgesbruno]

Tks @rmccue, I had asked about that on slack, but nobody has said that for me.
This is the new ticket #37090