Rollup merge of #143529 - pixel27:master, r=jhpratt

Renamed retain_mut to retain on LinkedList as mentioned in the ACP

This is for proposal: rust-lang/libs-team#250

The original check-in (rust-lang/rust#114136) contained both methods **retain** and **retain_mut**, which does not conform to rust-lang/libs-team#250 (comment).

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.

Author: jhpratt