Remove unnecessary import of `crate::marker` in std::sys_common::remutex.
Remove unnecessary import of `crate::marker` in std::sys_common::remutex.

It was used for marker::Send, but Send is already in scope.

graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust