MarkdownButton: don't do the availability check in the component itself

app/soapbox/features/compose/components/markdown_button.js

@@ -17,7 +17,6 @@ class MarkdownButton extends React.PureComponent {
     active: PropTypes.bool,
     onClick: PropTypes.func.isRequired,
     intl: PropTypes.object.isRequired,
-    unavailable: PropTypes.bool,
   };
 
   handleClick = () => {
@@ -25,11 +24,7 @@ class MarkdownButton extends React.PureComponent {
   }
 
   render() {
-    const { intl, active, unavailable } = this.props;
+    const { intl, active } = this.props;
-
-    if (unavailable) {
-      return null;
-    }
 
     return (
       <div className='compose-form__markdown-button'>

app/soapbox/features/compose/containers/markdown_button_container.js

@@ -1,17 +1,11 @@
 import { connect } from 'react-redux';
 
-import { getFeatures } from 'soapbox/utils/features';
-
 import { changeComposeContentType } from '../../../actions/compose';
 import MarkdownButton from '../components/markdown_button';
 
 const mapStateToProps = (state, { intl }) => {
-  const instance = state.get('instance');
-  const features = getFeatures(instance);
-
   return {
     active: state.getIn(['compose', 'content_type']) === 'text/markdown',
-    unavailable: !features.richText,
   };
 };