popovers: Remove "light-border" Tippyjs theme usage.

The "light-border" theme was set as the default theme for all the
popovers, but was no longer in use, since all of our current popovers
either use the "popover-menu" theme or the "dropdown-widget" theme.

In response to the above reasoning, this commit removes the redundant
"light-border" Tippyjs theme usage from the codebase.

web/src/bundles/app.ts

@@ -17,7 +17,6 @@ import "tippy.js/dist/tippy.css";
 // Adds color inheritance to the borders when using the default CSS Arrow.
 // https://atomiks.github.io/tippyjs/v6/themes/#arrow-border
 import "tippy.js/dist/border.css";
-import "tippy.js/themes/light-border.css";
 import "katex/dist/katex.css";
 import "flatpickr/dist/flatpickr.css";
 import "flatpickr/dist/plugins/confirmDate/confirmDate.css";

web/src/popover_menus.ts

@@ -196,10 +196,6 @@ export const default_popover_props: Partial<tippy.Props> = {
     trigger: "click",
     interactive: true,
     hideOnClick: true,
-    /* The light-border TippyJS theme is a bit of a misnomer; it
-       is a popover styling similar to Bootstrap.  We've also customized
-       its CSS to support Zulip's dark theme. */
-    theme: "light-border",
     // The maxWidth has been set to "none" to avoid the default value of 300px.
     maxWidth: "none",
     touch: true,

web/styles/dark_theme.css

@@ -27,17 +27,6 @@
         }
     }
 
-    /* Extend the 'light-border' TippyJS theme, which is intended for
-       popovers/menus that should use default background colors, to use
-       our dark theme colors in Zulip's dark theme.
-     */
-    .tippy-box[data-theme~="light-border"] {
-        .tippy-content a,
-        p {
-            color: hsl(236deg 33% 90%);
-        }
-    }
-
     .dropdown-list-delete {
         /* hsl(7deg 100% 74%) corresponds to var(--red-250) */
         color: color-mix(