alerts: Remove unused .alert-msg elements.

web/styles/dark_theme.css

@@ -409,10 +409,6 @@
         color: inherit !important;
     }
 
-    .private-message .alert-msg {
-        background-color: hsl(208deg 17% 29%);
-    }
-
     .active_private_messages_section {
         #private_messages_section,
         #private_messages_list,

web/styles/message_row.css

@@ -243,12 +243,6 @@ $time_column_min_width: 50px; /* + padding */
             margin-top: $distance_of_non_text_elements_from_message_box;
         }
 
-        .alert-msg {
-            /* Span both the controls and time columns */
-            grid-row: controls-start;
-            grid-column: controls-start / time-end;
-        }
-
         .message_length_controller {
             grid-area: more;
         }

web/styles/zulip.css

@@ -1161,16 +1161,6 @@ td.pointer {
     animation: rotate 1s infinite linear;
 }
 
-/* The way this overrides the menus with a background-color and a high
-   z-index is kinda hacky, and requires some annoying color-matching,
-   but it works. */
-.alert-msg {
-    color: hsl(170deg 48% 54%);
-    z-index: 999;
-    font-weight: 400;
-    display: none;
-}
-
 .status-time {
     top: 8px !important;
 }
@@ -1583,10 +1573,6 @@ td.pointer {
 .message_row.private-message {
     background-color: var(--color-background-private-message-content);
 
-    .alert-msg {
-        background-color: hsl(192deg 20% 95%);
-    }
-
     .date_row {
         background-color: var(--color-background-private-message-content);
     }

web/templates/message_body.hbs

@@ -26,8 +26,6 @@
     {{/if}}
 </span>
 
-<span class="alert-msg"></span>
-
 <a {{#unless msg/locally_echoed}}href="{{ msg/url }}"{{/unless}} class="message_time{{#if status_message}} status-time{{/if}}">
     {{#unless include_sender}}
     <span class="copy-paste-text">&nbsp;</span>