Replaced custom readSeekCloser with io.ReadSeekCloser.
Replaced custom readSeekCloser with io.ReadSeekCloser.

Go 1.16 has added a new io.ReadSeekCloser definition, so we no longer
need our custom interface definition.

Signed-off-by: Jacob Howard <f862f167b85d41b225785c70d70808bc7337c1fe@havoc.io>

havoc-io/mutagen,havoc-io/mutagen