Remove Custuomize Sections

The first thing to try is to increase the priority of your customize_register action from the default of 10 to something like 100. The reason for this is other components add their sections and controls in the same action and some after 10, so the things you are trying to remove may simply not have been added yet.

The second thing to note is that Widgets and Menus are a special case and they should not be disabled in this way. Instead, there is a dedicated filter you use to prevent them from getting loaded:

add_filter( 'customize_loaded_components', '__return_empty_array' );

For more see the hook docs.

See also my post on how to reset the Customizer to a blank slate.