daemon/graphdriver: remove redundant init()

This was added in a63ff8da46, but looks
like the only reason was to just have the var initialized, so let's
do so when we create the var.

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

daemon/graphdriver/driver.go

@@ -16,7 +16,7 @@ import (
 )
 
 // All registered drivers
-var drivers map[string]InitFunc
+var drivers = make(map[string]InitFunc)
 
 // CreateOpts contains optional arguments for Create() and CreateReadWrite()
 // methods.
@@ -111,10 +111,6 @@ type FileGetCloser interface {
 	Close() error
 }
 
-func init() {
-	drivers = make(map[string]InitFunc)
-}
-
 // Register registers an InitFunc for the driver.
 func Register(name string, initFunc InitFunc) error {
 	if _, exists := drivers[name]; exists {