Fix the code warnings when building

[hsluoyz]

Can we fix the following warnings?

  InternalEnforcer.cs(32, 48): [CS0168] The variable 'e' is declared but never used
  InternalEnforcer.cs(71, 48): [CS0168] The variable 'e' is declared but never used
  InternalEnforcer.cs(109, 48): [CS0168] The variable 'e' is declared but never used
  Config.cs(99, 36): [CS0168] The variable 'e' is declared but never used
  DefaultFileAdapter.cs(29, 32): [CS0168] The variable 'e' is declared but never used
  InternalEnforcer.cs(32, 48): [CS0168] The variable 'e' is declared but never used
  InternalEnforcer.cs(71, 48): [CS0168] The variable 'e' is declared but never used
  DefaultFileAdapter.cs(29, 32): [CS0168] The variable 'e' is declared but never used
  InternalEnforcer.cs(109, 48): [CS0168] The variable 'e' is declared but never used
  Config.cs(99, 36): [CS0168] The variable 'e' is declared but never used
  NuGet.Build.Tasks.Pack.targets(198, 5): [NU5125] The 'licenseUrl' element will be deprecated. Consider using the 'license' element instead.
  NuGet.Build.Tasks.Pack.targets(198, 5): [NU5048] The 'PackageIconUrl'/'iconUrl' element is deprecated. Consider using the 'PackageIcon'/'icon' element instead. Learn more at https://aka.ms/deprecateIconUrl

[hsluoyz]

@huazhikui @xcaptain @sagilio

[sagilio]

The warmings in InternalEnforcer.cs can be fixed when merged this PR Change update policy workflow. I will fix others at another PR.