This reverts the following commits:

- e98e8e64.
  "Documentation updates for changed `vcl_hash{}` / `hash_data()`"
- 001279eb.
  "Document proper design pattern for using hash_data() in vcl_recv,"
- e36573e2.
  "Add a test-case for hash_data() in vcl_recv{}"
- 03fe0cee.
  "Allow hash_data() in vcl_recv{}"
- 4ebc3cfe.
  "Make it possible to override the initial digest, and explain in"
- d6ad52f5
  "Change the way we calculate the hash key for the cache."

Conflicts:
	doc/sphinx/reference/dp_vcl_recv_hash.rst
	doc/sphinx/reference/index.rst

Concerns were raised regarding a change of the way we compute the hash
key outside of the dot-zero release where we would expect such breaking
changes (among other things, vmod_shard relies on hash stability).

There is also no definite consensus of how to handle hashing from
vcl_recv.