Delete tests and code for ecdsa over curve25519.

* ecc-eh-to-a.c (ecc_eh_to_a): Require op == 0, delete code only
used for non-standard ecdsa over curve25519.
* testsuite/ecdsa-sign-test.c (test_main): Delete test of ecdsa
over curve25519.
* testsuite/ecdsa-verify-test.c (test_main): Likewise.
* testsuite/ecdsa-keygen-test.c (test_main): Exclude curve25519
from test.