fix: rule of 3 for xtimit

[eric-wieser]

This PR adds a missing move assignment operator, and deletes the copy assignment operator.

C++ types should not implement move constructors without also implementing move assignment. This also ensures that m_fn is correctly emptied after a move, which is not guaranteed by the standard.

This change is also needed to allow lean::optional to be eventually replaced by std::optional.

[eric-wieser]

changelog-skip

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 4077bf2c05b74a2eaaf1db9be957236f29614f63 --onto 4ce7ad19cea348506876685a19a7e0cca442c6d0. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-17 13:13:05)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 4077bf2c05b74a2eaaf1db9be957236f29614f63 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-10-17 13:13:07)