HTTP parser: removed unused "exten" field.

This field was intended for MIME type lookup by file extension when serving
static files, but this use case is too narrow; only a fraction of requests
targets static content, and the URI presumably isn't rewritten.  Moreover,
current implementation uses the entire filename for MIME type lookup if the
file has no extension.

Instead of extracting filenames and extensions when parsing requests, it's
easier to obtain them right before serving static content; this behavior is
already implemented.  Thus, we can drop excessive logic from parser.

src/nxt_http_parse.h

@@ -44,7 +44,6 @@ struct nxt_http_request_parse_s {
 
     nxt_str_t                 path;
     nxt_str_t                 args;
-    nxt_str_t                 exten;
 
     nxt_http_ver_t            version;