Cleanup legacy digest methods.

Cleanup legacy digest methods.

Macros have been added to generate the simple legacy methods.
Engines and EVP_MD_METH_get methods still require access to the old legacy methods,
so they needed to be added back in.
They may only be removed after engines are deprecated and removed.
Removed some unnecessary #includes and #ifndef guards (which are done in build.info instead).

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10602)