nixos-rebuild --fast: Don't imply --show-trace

There is no logical reason for --fast to imply --show-trace, and this
seems to be a historical accident. Using --show-trace by default is
bad UX since it can give very long error messages (e.g. 550 lines for
a non-existent attribute in environment.systemPackages).

nixos/doc/manual/man-nixos-rebuild.xml

@@ -402,10 +402,9 @@
     </term>
     <listitem>
      <para>
-      Equivalent to <option>--no-build-nix</option>
-      <option>--show-trace</option>. This option is useful if you call
-      <command>nixos-rebuild</command> frequently (e.g. if you’re hacking on
-      a NixOS module).
+      Equivalent to <option>--no-build-nix</option>. This option is
+      useful if you call <command>nixos-rebuild</command> frequently
+      (e.g. if you’re hacking on a NixOS module).
      </para>
     </listitem>
    </varlistentry>

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -79,7 +79,6 @@ while [ "$#" -gt 0 ]; do
       --fast)
         buildNix=
         fast=1
-        extraBuildFlags+=(--show-trace)
         ;;
       --profile-name|-p)
         if [ -z "$1" ]; then