Only remove user agent if it matches.

In some cases we may want to provide a custom user agent

yt_dlp/networking/impersonate.py

@@ -131,7 +131,6 @@ def _get_impersonate_headers(self, request):
         headers = self._merge_headers(request.headers)
         if self._get_request_target(request) is not None:
             # remove all headers present in std_headers
-            headers.pop('User-Agent', None)
             for header in std_headers:
                 if header in headers and std_headers[header] == headers[header]:
                     headers.pop(header, None)