refactor: Add DelMutable() helper to unify post_updater.Run() + Del() pattern

[vyavdoshenko]

Follow-up: #5886

Introduces DbSlice::DelMutable() method to encapsulate the common pattern of running post_updater.Run() before deleting a key after FindMutable().

Note:
list_family.cc was not refactored as it uses a different pattern where deletion is conditional based on whether the list becomes empty, not unconditional.

Related to #5316

[BorysTheDev]

LGTM

[dranikpg]

You don't need to pass it by reference - ending its lifetime is more "idiomatically" correct. It's always the final action.

[vyavdoshenko]

You don't need to pass it by reference - ending it's lifetime is more "idiomatically" correct. It's always the final action.

Fixed