Use regular divs for Markdown buttons

The Markdown toggles need to watch `mousedown` to properly update state, so
let's drop back down to regular `div`s.

This also fixes element-hq/element-web#8866.

src/components/views/rooms/MessageComposer.js

@@ -500,7 +500,7 @@ export default class MessageComposer extends React.Component {
                     <div className="mx_MessageComposer_formatbar">
                         { formatButtons }
                         <div style={{ flex: 1 }}></div>
-                        <AccessibleButton className="mx_MessageComposer_formatbar_markdown mx_MessageComposer_markdownDisabled"
+                        <div className="mx_MessageComposer_formatbar_markdown mx_MessageComposer_markdownDisabled"
                             onMouseDown={this.onToggleMarkdownClicked}
                             title={_t("Markdown is disabled")}
                         />

src/components/views/rooms/MessageComposerInput.js

@@ -60,7 +60,6 @@ import ReplyPreview from "./ReplyPreview";
 import RoomViewStore from '../../../stores/RoomViewStore';
 import ReplyThread from "../elements/ReplyThread";
 import {ContentHelpers} from 'matrix-js-sdk';
-import AccessibleButton from '../elements/AccessibleButton';
 
 const EMOJI_UNICODE_TO_SHORTNAME = mapUnicodeToShort();
 const REGEX_EMOJI_WHITESPACE = new RegExp('(?:^|\\s)(' + asciiRegexp + ')\\s$');
@@ -1602,7 +1601,7 @@ export default class MessageComposerInput extends React.Component {
                     />
                 </div>
                 <div className={className}>
-                    <AccessibleButton className={markdownClasses}
+                    <div className={markdownClasses}
                         onMouseDown={this.onMarkdownToggleClicked}
                         title={this.state.isRichTextEnabled ? _t("Markdown is disabled") : _t("Markdown is enabled")}
                     />