Remove unused Elliptic Curve code.

Remove unused Elliptic Curve code.

For various reasons, the ecp_nistp* and ecp_nistz* code is unused. While
ecp_nistp* was being compiled, it is disabled due to
OPENSSL_NO_EC_NISTP_64_GCC_128 being defined. On the other hand,
ecp_nistz* was not even being built.

We will bring in new versions or alternative versions of such code, if we
end up enabling it in the future. For now it is just causing complexity
(and grep noise) while trying to improve the EC code.

Discussed with tb@