fix(diagnostic): remove unnecessary config lines

The following lines do nothing and can be removed:

 format = function(diagnostic)
   local diagnostic_message = {
     [vim.diagnostic.severity.ERROR] = diagnostic.message,
     [vim.diagnostic.severity.WARN] = diagnostic.message,
     [vim.diagnostic.severity.INFO] = diagnostic.message,
     [vim.diagnostic.severity.HINT] = diagnostic.message,
   }
   return diagnostic_message[diagnostic.severity]
 end,

The following line causes diagnostics which aren't of severity level
error to not have the specific text area underlined. This is less useful
than the default of showing underlines for all diagnostic levels, so
remove this line as well:

  underline = { severity = vim.diagnostic.severity.ERROR },

init.lua

@@ -631,7 +631,6 @@ require('lazy').setup({
       vim.diagnostic.config {
         severity_sort = true,
         float = { border = 'rounded', source = 'if_many' },
-        underline = { severity = vim.diagnostic.severity.ERROR },
         signs = vim.g.have_nerd_font and {
           text = {
             [vim.diagnostic.severity.ERROR] = '󰅚 ',
@@ -643,15 +642,6 @@ require('lazy').setup({
         virtual_text = {
           source = 'if_many',
           spacing = 2,
-          format = function(diagnostic)
-            local diagnostic_message = {
-              [vim.diagnostic.severity.ERROR] = diagnostic.message,
-              [vim.diagnostic.severity.WARN] = diagnostic.message,
-              [vim.diagnostic.severity.INFO] = diagnostic.message,
-              [vim.diagnostic.severity.HINT] = diagnostic.message,
-            }
-            return diagnostic_message[diagnostic.severity]
-          end,
         },
       }