Remove manual migrate command

Migration now happens automatically when the database schema is
updated (in Library._make_table()), so the manual 'beet migrate'
command is no longer needed.

Addresses PR review comment.

Author: dunkla

beets/ui/commands/__init__.py

@@ -24,7 +24,6 @@
 from .help import HelpCommand
 from .import_ import import_cmd
 from .list import list_cmd
-from .migrate import migrate_cmd
 from .modify import modify_cmd
 from .move import move_cmd
 from .remove import remove_cmd
@@ -53,13 +52,12 @@ def __getattr__(name: str):
     HelpCommand(),
     import_cmd,
     list_cmd,
-    migrate_cmd,
-    modify_cmd,
-    move_cmd,
+    update_cmd,
     remove_cmd,
     stats_cmd,
-    update_cmd,
     version_cmd,
+    modify_cmd,
+    move_cmd,
     write_cmd,
     config_cmd,
     completion_cmd,