Remove the separation betweeen enc_read_ctx and enc_write_ctx

Remove the separation betweeen enc_read_ctx and enc_write_ctx

Similarly with read_hash and write_hash. In the new model we have a
separate record layer object for reading and writing. Therefore we don't
need to distinguish between reading and writing inside the record layer
object in the encryption and md ctxs.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18132)