Fix not to force overwriting undo_ftplugin

[chitoku-k]

Fixed unnecessary overwrite by ftplugin. This meddled with other plugins as follows:

1. vim-toml initializes b:undo_ftplugin with an empty string
2. A plugin that appends a command to b:undo_ftplugin will insert | at the beginning of the string
3. The command will fail when it executes because it begins with | (invalid)

This variable should not be defined unless undo is required.

Cheers!

[cespare]

Thanks!