crypto: curve25519 - Remove unused kpp support

crypto: curve25519 - Remove unused kpp support

Curve25519 has both a library API and a crypto_kpp API.  However, the
crypto_kpp API for Curve25519 had no users outside crypto/testmgr.c.
I.e., no non-test code ever passed "curve25519" to crypto_alloc_kpp().

Remove this unused code.  We'll instead focus on the Curve25519 library
API (<crypto/curve25519.h>), which is a simpler and easier-to-use API
and is the API that is actually being used.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org> # m68k
Link: https://lore.kernel.org/r/20250906213523.84915-7-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>