NSIS cleanups and modernizations

[RedBlackAka]

Description

Minor NSIS cleanups and modernizations. No functional changes.

• Updated fallback version and year copyright

• Remove ifdef for 64-bit condition, as it now always is true

• Handle deletion of some stuff more elegantly

• Minor reorders

Motivation and Context

Outdated information and behavior from when there still was a 32-bit distribution. Also my former Start Menu cleanup was able to be handled with less lines.

How Has This Been Tested?

This has not been tested. Does not bring behavioral changes.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[RytoEX]

Seems fine.