crypto: testmgr - Remove curve25519 kpp tests

crypto: testmgr - Remove curve25519 kpp tests

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 tests for the "curve25519" kpp from crypto/testmgr.c.

Note that these tests just duplicated lib/crypto/curve25519-selftest.c,
which uses the same list of test vectors.  So they didn't really provide
any additional value.

Link: https://lore.kernel.org/r/20250906213523.84915-6-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>