Remove restic.Cache interface

It was used in one code path, which then asserted its concrete type as
*cache.Cache.

Privatised some of the interface methods.

internal/restic/cache.go

@@ -1,37 +0,0 @@
-package restic
-
-import "io"
-
-// Cache manages a local cache.
-type Cache interface {
-	// BaseDir returns the base directory of the cache.
-	BaseDir() string
-
-	// Wrap returns a backend with a cache.
-	Wrap(Backend) Backend
-
-	// IsNotExist returns true if the error was caused by a non-existing file.
-	IsNotExist(err error) bool
-
-	// Load returns a reader that yields the contents of the file with the
-	// given id if it is cached. rd must be closed after use. If an error is
-	// returned, the ReadCloser is nil. The files are still encrypted
-	Load(h Handle, length int, offset int64) (io.ReadCloser, error)
-
-	// SaveIndex saves an index in the cache.
-	Save(Handle, io.Reader) error
-
-	// SaveWriter returns a writer for the to be cached object h. It must be
-	// closed after writing is finished.
-	SaveWriter(Handle) (io.WriteCloser, error)
-
-	// Remove deletes a single file from the cache. If it isn't cached, this
-	// functions must return no error.
-	Remove(Handle) error
-
-	// Clear removes all files of type t from the cache that are not contained in the set.
-	Clear(FileType, IDSet) error
-
-	// Has returns true if the file is cached.
-	Has(Handle) bool
-}