Replace Bitstream Vera fonts by DejaVu in defaults

The default configuration installed the Bitstream Vera fonts, but DejaVu
is a superior replacement, and the default Fontconfig settings need it
now for the generic faces monospace, sans-serif, and serif.

nixos/modules/config/fonts/fonts.nix

···
       [ pkgs.xorg.fontbhttf
         pkgs.xorg.fontbhlucidatypewriter100dpi
         pkgs.xorg.fontbhlucidatypewriter75dpi
-        pkgs.ttf_bitstream_vera
+        pkgs.dejavu_fonts
         pkgs.freefont_ttf
         pkgs.liberation_ttf
         pkgs.xorg.fontbh100dpi