Remove unnecessary NRT directives from TaskDialog interop files

[kpreisser]

Remove now unnecessary NRT directives that were added by #1133, since #3451 enabled nullable reference types for System.Windows.Forms.Primitives globally in the project file.

Proposed changes

• Remove remaining #nullable enable directives in System.Windows.Forms.Primitives files.

Customer Impact

• None

Regression?

• No

Risk

Test methodology

• Changed one of the field types in PFTASKDIALOGCALLBACK to object? and verified that the compiler doesn't raise CS8632.

Test environment(s)

.NET SDK (reflecting any global.json):
Version: 5.0.100-rc.1.20367.2
Commit: 0dd3ff77c7

Runtime Environment:
OS Name: Windows
OS Version: 10.0.18363
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\5.0.100-rc.1.20367.2\

Host (useful for support):
Version: 5.0.0-preview.8.20361.2
Commit: f37dd6fc85

.NET SDKs installed:
3.1.400-preview-015203 [C:\Program Files\dotnet\sdk]
5.0.100-rc.1.20367.2 [C:\Program Files\dotnet\sdk]

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

Merging #3602 into master will decrease coverage by 31.66996%.
The diff coverage is n/a.

@@                 Coverage Diff                  @@
##              master       #3602          +/-   ##
====================================================
- Coverage   67.04925%   35.37929%   -31.66997%     
====================================================
  Files           1364         905         -459     
  Lines         503861      250771      -253090     
  Branches       40782       36601        -4181     
====================================================
- Hits          337835       88721      -249114     
+ Misses        160459      157204        -3255     
+ Partials        5567        4846         -721     

Flag	Coverage Δ
#Debug	35.37929% <ø> (-31.66997%)	⬇️
#production	35.37929% <ø> (ø)
#test	?

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[JeremyKuhne]

Thanks @kpreisser