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 also serves little purpose, seemingly only reducing
the spacing between the end of lines and diagnostic virtual text by two
characters, so it can also be removed:

  spacing = 2,

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] = '󰅚 ',
@@ -642,16 +641,6 @@ require('lazy').setup({
         } or {},
         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,
         },
       }