layer: remove unused ErrActiveMount, ErrNotMounted, ErrNotSupported

These errors were added in 500e77bad0b19b3b1c8e6ac195485adcb70daef1, but were
never used.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

layer/layer.go

@@ -38,23 +38,10 @@ var (
 	// used for creation.
 	ErrMountNameConflict = errors.New("mount already exists with name")
 
-	// ErrActiveMount is used when an operation on a
-	// mount is attempted but the layer is still
-	// mounted and the operation cannot be performed.
-	ErrActiveMount = errors.New("mount still active")
-
-	// ErrNotMounted is used when requesting an active
-	// mount but the layer is not mounted.
-	ErrNotMounted = errors.New("not mounted")
-
 	// ErrMaxDepthExceeded is used when a layer is attempted
 	// to be created which would result in a layer depth
 	// greater than the 125 max.
 	ErrMaxDepthExceeded = errors.New("max depth exceeded")
-
-	// ErrNotSupported is used when the action is not supported
-	// on the current host operating system.
-	ErrNotSupported = errors.New("not support on this host operating system")
 )
 
 // ChainID is the content-addressable ID of a layer.