Remove kx-safe-default gate on SslCurve

[rushilmehra]

While setting curves should be restricted by the kx-safe-default feature, reading the curve is allowed.

[cjpatton]

Exposing the SSL curves list is fine, but we still want to prevent the user from setting the curves list if the "kx-*" feature flags are in use. This is because the features will silently overwrite the user's preference.