Remove FdRef definition
Remove FdRef definition

Support for AsRawFd was added to RawFd in rust 1.48, our lowest supported
toolchain is rust 1.49, so we can drop FdRef.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd