fix: remove unnecessary diagnostic config lines

[ronandalton]

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 },

[guru245]

I agree with the first one. But IMHO the second and the third one are a matter of preference and so we don't really need to change.

[ronandalton]

I somewhat agree, but if it's a matter of preference then I think it shouldn't be in kickstart to begin with. There should be a good usability/QoL reason to deviate from the default nvim behavior and increase the config size, and I'm not seeing it. The underline config makes the experience worse than the default because now you can't tell which part of the line has the issue. The spacing = 2 line can stay if that's what people want - I don't think that makes much difference either way.