Renamed retain_mut to retain on LinkedList as mentioned in the ACP
This is for proposal: https://github.com/rust-lang/libs-team/issues/250
The original check-in (https://github.com/rust-lang/rust/pull/114136) contained both methods **retain** and **retain_mut**, which does not conform to https://github.com/rust-lang/libs-team/issues/250#issuecomment-1766822671.
I updated the retain documentation to specify **&mut e**, removed the **retain** method and renamed **retain_mut** to **retain** to conform to the request.
The pull request doesn't really contain much that is new, just removes the unwanted method to meet the requirements.
I've run the tests "library/alloc" on the code and no issues.
Hopefully I'm not stepping on the original author's toes. I just ran across a need for the method and wondered why it was unstable.
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