Invoke-WinUtilISOScript.ps1's PostInstall script set DisableFileSyncNGSC
to 1, which FirstLogon.ps1 (from #4409) then immediately overrides back
to 0. The final value was correct, but only because one script undoes
the other's write on every install.
Removes the redundant PostInstall write so FirstLogon.ps1's existing
0 assignment is the only place setting this value.
Verified the script still parses correctly after the change.
