Mop up EC_GROUP precomp machinery.

Mop up EC_GROUP precomp machinery.

Since there are now no EC implementations that perform pre-computation at
the EC_GROUP level, remove all of the precomp machinery, including the
extra_data EC_GROUP member.

The ec_wNAF_mul() code is horrific - simply cut out the precomp code,
rather than trying to rewrite it (that's a project for another day).

ok tb@