Simplify http_parse_querystring()

Instead of reimplementing URL parameter parsing from scratch, use
url_decode_parameter_name() and url_decode_parameter_value() which are
already provided by Git.

Also, change the return type of http_parse_querystring() to void since
its only caller already ignores the return value.

Signed-off-by: Lukas Fleischer <lfleischer@lfos.de>