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_unit_field.h

@@ -12,7 +12,6 @@
 #include "nxt_unit_sptr.h"
 
 enum {
-    NXT_UNIT_HASH_HOST           = 0xE6EB,
     NXT_UNIT_HASH_CONTENT_LENGTH = 0x1EA0,
     NXT_UNIT_HASH_CONTENT_TYPE   = 0x5F7D,
     NXT_UNIT_HASH_COOKIE         = 0x23F2,