fix(snippet): do not add extra indent on newlines (#28538)

Reverts parts of https://github.com/neovim/neovim/pull/27674

LSP snippets typically do include tabs or spaces to add extra
indentation and don't rely on the client using `autoindent`
functionality.

For example:

    public static void main(String[] args) {\n\t${0}\n}

Notice the `\t` after `{\n`

Adding spaces or tabs independent of that breaks snippets for languages
like Haskell where you can have snippets like:

    ${1:name} :: ${2}\n${1:name} ${3}= ${0:undefined}

To generate:

    name ::
    name = undefined

runtime/lua/vim/snippet.lua

@@ -459,8 +459,7 @@ function M.expand(input)
       end
       -- Add the base indentation.
       if i > 1 then
-        line = #line ~= 0 and base_indent .. line
-          or (expandtab and (' '):rep(shiftwidth) or '\t'):rep(vim.fn.indent('.') / shiftwidth + 1)
+        line = base_indent .. line
       end
       lines[#lines + 1] = line
     end