avformat/avio: Remove no-op code in url_find_protocol().

In url_find_protocol(), proto_str is either "file" or a string
consisting of only the characters in URL_SCHEME_CHARS, which does not
include ','. Therefore the strchr(proto_str, ',') call always returns
NULL.

Note: The code was added in commit
6161c418.

Signed-off-by: Wan-Teh Chang <wtc@google.com>
Signed-off-by: Muhammad Faiz <mfcc64@gmail.com>