crypto: arm/curve25519 - Remove unused kpp support

crypto: arm/curve25519 - Remove unused kpp support

Curve25519 is used only via the library API, not the crypto_kpp API.  In
preparation for removing the unused crypto_kpp API for Curve25519,
remove the unused "curve25519-neon" kpp algorithm.

Note that the underlying NEON optimized Curve25519 code remains fully
supported and accessible via the library API.

It's also worth noting that even if the kpp support for Curve25519 comes
back later, there is no need for arch-specific kpp glue code like this,
as a single kpp algorithm that wraps the library API is sufficient.

Link: https://lore.kernel.org/r/20250906213523.84915-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>