The points are only converted to affine form when there are at least
three points being multiplied (in addition to the generator), but there
never is more than one point, so this is all dead code.

Also, I doubt that the comments "...point at infinity (which normally
shouldn't happen)" in the deleted code are accurate. And, the
projective->affine conversions that were removed from p224-64.c and
p256-64.c didn't seem to properly account for the possibility that any of
those points were at infinity.

Change-Id: I611d42d36dcb7515eabf3abf1857e52ff3b45c92
Reviewed-on: https://boringssl-review.googlesource.com/7100
Reviewed-by: David Benjamin <davidben@google.com>

This is taken from upstream, although it originally came from us. This
will only take effect on 64-bit systems (x86-64 and aarch64).

Before:

Did 1496 ECDH P-256 operations in 1038743us (1440.2 ops/sec)
Did 2783 ECDSA P-256 signing operations in 1081006us (2574.5 ops/sec)
Did 2400 ECDSA P-256 verify operations in 1059508us (2265.2 ops/sec)

After:

Did 4147 ECDH P-256 operations in 1061723us (3905.9 ops/sec)
Did 9372 ECDSA P-256 signing operations in 1040589us (9006.4 ops/sec)
Did 4114 ECDSA P-256 verify operations in 1063478us (3868.4 ops/sec)

Change-Id: I11fabb03239cc3a7c4a97325ed4e4c97421f91a9