Remove over-eager ftplugin indent settings

It's not really appropriate for filetype plugins to set indent-related options.

In this case, the appropriate options (autoindent, expandtab, and sw=2)
are already being set in indent/slim.vim.

smartindent is meant for C-like languages where the normal C indentation
doesn't work. There's no reason to use it here (autoindent is what you
want, but that's already set in indent/slim.vim).

There's no reason to set all of tabstop, softtabstop, and shiftwidth;
and when using smarttab, only shiftwidth is required.

However, smarttab is a global option with no buffer-local value, so
setting it (even with setlocal) changes the option for all buffers. Thus
it should be left up to the user.
Use setfiletype

This is the standard way to set the filetype, and allows syntax.vim to
set the correct syntax highlighting automatically.
Merge pull request #36 from jacknagel/patch-1

Remove over-eager ftplugin indent settings