net: nbuf: Remove unused net_nbuf_push() API

The net_nbuf_push() API is not used by anyone. Semantics are not
clear and following patch requires changes to push api, so removing
this API for now. If needed this can be re-introduced later.

Change-Id: I1d669c861590aa9bc80cc1ccb08144bd6020dac5
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>