Fix sanitize_callback for the container type setting and the navbar type setting

[IanDelMar]

Description

This PR

• removes the sanitize_callback argument from WP_Customize_Control which does not have such an argument.
• sets understrap_customize_sanitize_select to be the sanitize_callback when adding the setting.

Motivation and Context

Misplacement of the sanitize_callback => 'understrap_customize_sanitize_select'.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I pulled my branch from develop.
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• I have commented my code, particularly in hard-to-understand areas.
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