fix: return an error instead of panicking on orphaned MemMapFs Remove

unRegisterWithParent called log.Panic when a file's parent directory was
no longer registered (for example when the parent was removed first),
crashing the whole program from an ordinary Remove call. Return a normal
error instead so Remove, RemoveAll and Rename can surface it to the
caller. The now-unused log import is dropped.

Fixes #388

memmap.go

@@ -16,7 +16,6 @@ package afero
 import (
 	"fmt"
 	"io"
-	"log"
 	"os"
 	"path/filepath"
 	"sort"
@@ -71,7 +70,11 @@ func (m *MemMapFs) unRegisterWithParent(fileName string) error {
 	}
 	parent := m.findParent(f)
 	if parent == nil {
-		log.Panic("parent of ", f.Name(), " is nil")
+		// The parent directory is no longer registered (for example it was
+		// removed before this file). There is nothing to unregister from, and
+		// crashing the whole process is never appropriate, so surface an error
+		// that callers can handle.
+		return fmt.Errorf("parent of %s is nil", f.Name())
 	}
 
 	parent.Lock()