Fixed processing of SERVER_NAME after 77aad2c142a0.

Previously, the nxt_router_prepare_msg() function expected server host among
other headers unmodified.  It's not true anymore since normalization of the
Host header has been introduced in 77aad2c142a0.

The nxt_unit_split_host() function was removed.  It didn't work correctly with
IPv6 literals.  Anyway, after 77aad2c142a0 the port splitting is done in router
while Host header processing.

src/nxt_application.h

@@ -95,13 +95,13 @@ typedef struct {
     nxt_str_t                  version;
     nxt_str_t                  path;
     nxt_str_t                  query;
+    nxt_str_t                  server_name;
 
     nxt_list_t                 *fields;
 
     nxt_str_t                  cookie;
     nxt_str_t                  content_length;
     nxt_str_t                  content_type;
-    nxt_str_t                  host;
 
     off_t                      parsed_content_length;
     nxt_bool_t                 done;