Remove unused import

Author: davep

src/textual/widgets/_markdown.py

@@ -18,7 +18,7 @@
 from ..containers import Horizontal, Vertical, VerticalScroll
 from ..events import Mount
 from ..message import Message
-from ..reactive import Reactive, reactive, var
+from ..reactive import reactive, var
 from ..widget import Widget
 from ..widgets import Static, Tree