Remove the now redundant username color variables

They are replaced by the Compound theming options (specifically, username colors can be themed by changing the color of Compound's decorative color tokens).

res/themes/light-custom/css/_custom.pcss

@@ -32,9 +32,6 @@ $background: var(--background, $background);
 $panels: var(--panels, var(--cpd-color-gray-600));
 $panel-actions: var(--panels-actions, var(--cpd-color-gray-300));
 
-/* --accent-color */
-$username-variant3-color: var(--accent-color);
-
 /* --timeline-background-color */
 $button-secondary-bg-color: var(--timeline-background-color);
 $lightbox-border-color: var(--timeline-background-color);
@@ -110,14 +107,6 @@ $accent-alt: var(--primary-color);
 /* --warning-color */
 $button-danger-disabled-bg-color: var(--warning-color-50pct); /* still needs alpha at 0.5 */
 
-/* --username colors (which use a 0-based index) */
-$username-variant1-color: var(--username-colors_0, $username-variant1-color);
-$username-variant2-color: var(--username-colors_1, $username-variant2-color);
-$username-variant3-color: var(--username-colors_2, $username-variant3-color);
-$username-variant4-color: var(--username-colors_3, $username-variant4-color);
-$username-variant5-color: var(--username-colors_4, $username-variant5-color);
-$username-variant6-color: var(--username-colors_5, $username-variant6-color);
-
 /* --timeline-highlights-color */
 $event-selected-color: var(--timeline-highlights-color);
 $event-highlight-bg-color: var(--timeline-highlights-color);