Eliminate unnecessary destructor, copy and assignment ops in ofColor.

ofColor does not need these operations to be explicitly defined because the
compiler-supplied versions should be sufficient. Further, defining them
explicitly (and making them not inline) will hurt performance by requiring
unnecessary function calls.

Removing these also makes the class trivially copyable, which is also desirable
(it makes it possible to copy arrays of ofColor with memcpy instead of
individually copying each element, for example).
Merge pull request #3420 from nneonneo/ofColor-fixes

Eliminate unnecessary destructor, copy and assignment ops in ofColor.