Hello Yoshihiko,
My mistake! I tried it on a TWX 9.3.4 instance because the widget looked the same. But this is not the case. It's a slightly different widget and has a different structure. I've now tried it on a ThingWorx 9.2.1-b112 instance and hope the new CSS works for you now.
Edit: No you don´t need to do any further configurations on the DisplayValue Widget itself. If you want to apply the css only on this Widget, consider adding a ".exampleNameClass" before my provided classes.

Add following CSS, if you want to ...
/*hide only the disclosure-button*/
ptcs-value-display::part(item-value)::part(disclosure-button){
/*visibility:hidden !important;*/
display: none !important;
}

/*To hide the whole disclosure container */
ptcs-value-display::part(item-value)::part(disclosure-button-container){
/*visibility:hidden !important;*/
display: none !important;
/*height: 10px !important;*/
}

/*Hide the border from the whole widget*/
ptcs-value-display::part(item-value)::part(popup-root){
border: 0 !important;
}
If you don´t want to remove the border, simply use the "visibility:hidden !important;" and the "height: 10px !important;" properties and remove the "display: none !important;".
Best regards
Markus