vi-mode: remove line-init/finish widgets that broken terminfo keybindings

The zle-line-init and zle-line-finish definitions here were broken with
respect to smkx/rmkx because their "if" logic had fallthrough where it shouldn't,
so the mode was left in rmkx all the time. This just removes those widgets
entirely, because they're now defined (correctly) in lib/keybindings.zsh and
not needed in plugins.