Re-disables enableScalarColumnContextMenus flag (tensorflow#6838)

## Motivation for features / changes
In 3p, enabling standard (non-hparam column) hide is currently dangerous
as there is currently no way to re-show them. Enabling for 3p will have
to wait until we add an affordance other than the 1p-only column
customizer.

Affected users can clear localStorage for the page, or add the
`enableScalarColumnCustomization=true` flag to enable the column
customizer in the settings menu and re-add hidden columns.

## Technical description of changes
Sets `enableScalarColumnContextMenus` default to `false`. (Reverts
tensorflow#6817)

Author: hoonji

tensorboard/webapp/feature_flag/store/feature_flag_metadata.ts

@@ -111,7 +111,7 @@ export const FeatureFlagMetadataMap: FeatureFlagMetadataMapType<FeatureFlags> =
       parseValue: parseBoolean,
     },
     enableScalarColumnContextMenus: {
-      defaultValue: true,
+      defaultValue: false,
       queryParamOverride: 'enableScalarColumnContextMenus',
       parseValue: parseBoolean,
     },