Stop passing "force" option when invoking cros-disks's Unmount method.

cros-disks's Unmount method now ignores the "force" option (CL:388592,
chromium:646224). This CL updates CrosDisksClient to stop passing the
"force" option when invoking the Unmount method.

BUG=700548
TEST=chromos_unittests

Review-Url: https://codereview.chromium.org/2743113003
Cr-Commit-Position: refs/heads/master@{#456230}

chromeos/dbus/cros_disks_client.cc

@@ -40,10 +40,6 @@ const char kReadWriteOption[] = "rw";
 const char kRemountOption[] = "remount";
 const char kMountLabelOption[] = "mountlabel";
 
-const char* kDefaultUnmountOptions[] = {
-  "force",
-};
-
 const char kLazyUnmountOption[] = "lazy";
 
 // Checks if retrieved media type is in boundaries of DeviceMediaType.
@@ -133,9 +129,7 @@ class CrosDisksClientImpl : public CrosDisksClient {
     dbus::MessageWriter writer(&method_call);
     writer.AppendString(device_path);
 
-    std::vector<std::string> unmount_options(
-        kDefaultUnmountOptions,
-        kDefaultUnmountOptions + arraysize(kDefaultUnmountOptions));
+    std::vector<std::string> unmount_options;
     if (options == UNMOUNT_OPTIONS_LAZY)
       unmount_options.push_back(kLazyUnmountOption);