fix format_on_save configuration

it actually works properly for selectively enabling/disabling
format on save via conform based on filetype.

i apparently misunderstood how lua tables work and checking if values
exist in them or not.

lua/plugins/conform.lua

@@ -16,22 +16,18 @@ return {
     opts = {
       notify_on_error = false,
       format_on_save = function(bufnr)
-        -- Filetypes to apply formatting to on save. Remove from this list
-        -- to disable formatting on save for that filetype.
-        local filetypes = {
-          'lua',
-          'rust',
+        local autoformat_filetypes = {
+          lua = true,
+          rust = true,
         }
-        local lsp_format_opt
-        if filetypes[vim.bo[bufnr].filetype] then
-          lsp_format_opt = 'fallback'
+        if autoformat_filetypes[vim.bo[bufnr].filetype] then
+          return {
+            timeout_ms = 500,
+            lsp_format = 'fallback',
+          }
         else
-          lsp_format_opt = 'never'
+          return
         end
-        return {
-          timeout_ms = 500,
-          lsp_format = lsp_format_opt,
-        }
       end,
       formatters_by_ft = {
         lua = { 'stylua' },