Remove include/internal/decoder.h, as it's superfluous

Remove include/internal/decoder.h, as it's superfluous

The routines declared in there are entirely libcrypto internal, so
include/crypto/decoder.h is better suited for them.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/21733)