nixos/timesyncd: remove unnecessary workaround

This workardoung is now unnecessary because systemd and
systemd-timesyncd will never actually revert to a time lower than the
current system time. It will only *advance* the system time to the EPOCH
if the system time is lower.

This also makes systemd-timesyncd bashless as we remove the preStart
script for any NixOS version newer than 19.09.

nixos/modules/system/boot/timesyncd.nix

@@ -75,28 +75,16 @@ in
       # NSS module path so that systemd-timesyncd keeps using other NSS modules that are configured in the system.
       environment.LD_LIBRARY_PATH = config.system.nssModules.path;
 
-      preStart = (
-        # Ensure that we have some stored time to prevent
-        # systemd-timesyncd to resort back to the fallback time.  If
-        # the file doesn't exist we assume that our current system
-        # clock is good enough to provide an initial value.
-        ''
-          if ! [ -f /var/lib/systemd/timesync/clock ]; then
-            test -d /var/lib/systemd/timesync || mkdir -p /var/lib/systemd/timesync
-            touch /var/lib/systemd/timesync/clock
+      preStart =
+        # workaround an issue of systemd-timesyncd not starting due to upstream systemd reverting their dynamic users changes
+        #  - https://github.com/NixOS/nixpkgs/pull/61321#issuecomment-492423742
+        #  - https://github.com/systemd/systemd/issues/12131
+        lib.optionalString (versionOlder config.system.stateVersion "19.09") ''
+          if [ -L /var/lib/systemd/timesync ]; then
+            rm /var/lib/systemd/timesync
+            mv /var/lib/private/systemd/timesync /var/lib/systemd/timesync
           fi
-        ''
-        +
-          # workaround an issue of systemd-timesyncd not starting due to upstream systemd reverting their dynamic users changes
-          #  - https://github.com/NixOS/nixpkgs/pull/61321#issuecomment-492423742
-          #  - https://github.com/systemd/systemd/issues/12131
-          (lib.optionalString (versionOlder config.system.stateVersion "19.09") ''
-            if [ -L /var/lib/systemd/timesync ]; then
-              rm /var/lib/systemd/timesync
-              mv /var/lib/private/systemd/timesync /var/lib/systemd/timesync
-            fi
-          '')
-      );
+        '';
     };
 
     environment.etc."systemd/timesyncd.conf".text = ''