net: lib: http_server: Simplify HTTP2 frame header parsing

* Remove unneeded variable.
* Use system utilities to read big endian numbers instead of parsing
  manually.
* Remove `payload` member from the http_frame structure. It's not used
  for anything useful, and could actually be misleading, as in case of
  large frames, where not entire frame is parsed at once it will point
  to incorrect location.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>