bootstrap: Clean up .close references.

Removing `.alert` from the dark theme seems acceptable,
as there is nowhere in the codebase we're setting a
`text-shadow` value that would require `none` here.

web/styles/dark_theme.css

@@ -188,10 +188,6 @@
         opacity: 0.2;
     }
 
-    .close {
-        color: hsl(236deg 33% 80%);
-    }
-
     .zoom-in {
         #topics_header {
             background-color: var(--color-background);
@@ -293,19 +289,6 @@
         border: 1px solid hsl(11deg 46% 54%);
     }
 
-    .alert {
-        text-shadow: none;
-
-        .close {
-            color: inherit;
-            opacity: 0.8;
-        }
-
-        .close:hover {
-            opacity: 1;
-        }
-    }
-
     .alert.alert-success {
         color: inherit;
         background-color: hsl(161deg 60% 46% / 20%);

web/third/bootstrap/css/bootstrap-alert.css

@@ -18,12 +18,6 @@
 .alert h4 {
   margin: 0;
 }
-.alert .close {
-  position: relative;
-  top: -2px;
-  right: -21px;
-  line-height: 20px;
-}
 .alert-success {
   background-color: #dff0d8;
   border-color: #d6e9c6;

web/third/bootstrap/css/bootstrap.app.css

@@ -101,26 +101,3 @@ input:focus:invalid:focus {
   -moz-box-shadow: 0 0 6px #f8b9b7;
   box-shadow: 0 0 6px #f8b9b7;
 }
-.close {
-  float: right;
-  font-size: 20px;
-  font-weight: bold;
-  line-height: 20px;
-  color: #000000;
-  text-shadow: 0 1px 0 #ffffff;
-  opacity: 0.2;
-}
-.close:hover,
-.close:focus {
-  color: #000000;
-  text-decoration: none;
-  cursor: pointer;
-  opacity: 0.4;
-}
-button.close {
-  padding: 0;
-  cursor: pointer;
-  background: transparent;
-  border: 0;
-  -webkit-appearance: none;
-}