Remove underline from gutter line numbers (nordtheme#185)

Vim version 8.1.2029 [1] added the `underline` attribute for the
`CursorLineNr` group to `cterm` [2] based on vim/vim#4933 [3].
This change resulted in gutter line numbers being underlined which has
now been reverted back to Nord's style by explicitly setting the
attribute for the group to `NONE`.

[1]: https://github.com/vim/vim/releases/tag/v8.1.2029
[2]: vim/vim@d9b0d83...017ba07#diff-80fffb3e9c20e93e5b2328a9a20e19c9
[3]: vim/vim#4933

Resolves nordthemeGH-174

colors/nord.vim

@@ -219,9 +219,9 @@ endif
 "+--- Gutter ---+
 call s:hi("CursorColumn", "", s:nord1_gui, "NONE", s:nord1_term, "", "")
 if g:nord_cursor_line_number_background == 0
-  call s:hi("CursorLineNr", s:nord4_gui, s:nord0_gui, "NONE", "", "", "")
+  call s:hi("CursorLineNr", s:nord4_gui, s:nord0_gui, "NONE", "", "NONE", "")
 else
-  call s:hi("CursorLineNr", s:nord4_gui, s:nord1_gui, "NONE", s:nord1_term, "", "")
+  call s:hi("CursorLineNr", s:nord4_gui, s:nord1_gui, "NONE", s:nord1_term, "NONE", "")
 endif
 call s:hi("Folded", s:nord3_gui, s:nord1_gui, s:nord3_term, s:nord1_term, s:bold, "")
 call s:hi("FoldColumn", s:nord3_gui, s:nord0_gui, s:nord3_term, "NONE", "", "")