build: Remove references to the 'nix-hash' program.

* configure.ac: Remove check for 'nix-hash'.
* tests/base32.scm (%nix-hash, %have-nix-hash?): Remove.
("sha256 & bytevector->nix-base32-string"): Remove test.
* build-aux/pre-inst-env.in: Do not set 'NIX_HASH' environment
variable.