tests/extmod: Remove conditional import of uos_vfs, it no longer exists.

This conditional import was only used to get the tests working on the unix
coverage build, which has now switched to use VFS by default so the uos
module alone has the required functionality.

tests/extmod/vfs_basic.py

@@ -1,11 +1,7 @@
 # test VFS functionality without any particular filesystem type
 
 try:
-    try:
-        import uos_vfs as uos
-        open = uos.vfs_open
-    except ImportError:
-        import uos
+    import uos
     uos.mount
 except (ImportError, AttributeError):
     print("SKIP")