last `unmap` can cause the error "E31: No such mapping" when
`doaudocmd FileType go` if appending other commands to `b:undo_ftplugin` i.e.
the space and the next bar as `let b:undo_ftplugin .= ' | setl ...'`.
closes: vim/vim#17664f9d87fa6ba
Co-authored-by: ichizok <gclient.gaap@gmail.com>