This commit fixes the opacity for curves hiding the option.
Actually, the curve points and handles drawing is using the same code that mesh curves and the opacity is not supported. While this feature will be added for mesh curves and gpencil, now it's better to hide this option.
Reviewed: Matias Mendiola
Note: The handle problem reported in this task was fixed in a separated commit: 203e7ba332
2b8e35eeb0