Don't use custom CSS for announcement banner

Author: dstansby

doc/_static/mpl.css

@@ -14,29 +14,6 @@
     margin: 0;
 }
 
-/* Make announcement an error colour for unreleased documentation, and sticky. */
-#unreleased-message.bd-header-announcement {
-    border-bottom: solid var(--pst-color-danger-highlight);
-    color: var(--pst-color-danger-text);
-    font-weight: var(--pst-admonition-font-weight-heading);
-    position: sticky;
-    top: 0;
-    z-index: 1050;
-}
-
-#unreleased-message.bd-header-announcement:after {
-    background-color: var(--pst-color-danger);
-    opacity: 1;
-}
-
-#unreleased-message.bd-header-announcement a {
-    color: var(--pst-color-danger-text);
-}
-
-#unreleased-message.bd-header-announcement + .bd-navbar {
-    top: 3rem;  /* Minimum height of announcement header. */
-}
-
 /* multi column TOC */
 .contents ul {
     list-style-type: none;