Revert "add underline styles to admonition content (#5435)" (#5436)

This reverts commit 8a7fbb5.

src/css/custom.scss

@@ -160,34 +160,25 @@ h4 {
   padding-bottom: 0;
 }
 
-// distribute icons beside search input more evenly
 [class^='toggle_'] button {
   margin-left: 0.15rem;
 }
 
+.alert.alert--info {
+  margin-bottom: 2rem;
+}
+
 /* using attribute selector for these dynamic classes */
 [class^='admonitionHeading'] {
   display: none;
 }
 
-.alert.alert--info {
-  margin-bottom: 2rem;
-}
-
 .alert.alert--info h2,
 h3 {
   border-bottom: none;
   padding-bottom: 0;
 }
 
-// ensure text links in admonitions have underline
-.alert.alert--info a {
-  border-bottom: 1px dotted var(--ifm-link-color);
-  &:hover {
-    border-bottom: 1px solid var(--ifm-link-color);
-  }
-}
-
 div[class^='announcementBar'] {
   font-size: 1rem;
   font-weight: var(--ifm-font-weight-semibold);