Remove unnecessary copy constructor (#83030)
Re-land of https://github.com/pytorch/pytorch/pull/82626 which somehow broke deploy build
Fixes annoying warnings when building with clang:
```
../c10/util/variant.h:2256:9: warning: definition of implicit copy constructor for 'impl<c10::Scalar, c10::basic_string_view<char>>' is deprecated because it has a user-declared copy assignment operator [-Wdeprecated-copy]
impl& operator=(const impl& other) = default;
^
```
Preliminary step for switching MacOS builds to ones with `-Werror`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83030
Approved by: https://github.com/kit1980, https://github.com/seemethere
