fscrypt: remove unused Speck definitions

These old unused definitions were originally left around to prevent the
same mode numbers from being reused.  However, we've now decided to
reuse the mode numbers anyway.  So let's completely remove these old
unused definitions to avoid confusion.  There is no reason for any code
to be using these constants in any way; and indeed, Debian Code Search
shows no uses of them (other than in copies or translations of the
header).  So this should be perfectly safe.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20221202035529.55992-1-ebiggers@kernel.org