ICU-22532 Remove redundant 'void' from empty C++ parameter lists

[roubert]

Follow-up to PR #2744, we might as well be consistent about this.

https://releases.llvm.org/16.0.0/tools/clang/tools/extra/docs/clang-tidy/checks/modernize/redundant-void-arg.html

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22532
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[FrankYFTang]

Could you point out how you generate this PR by tool so we can learn from that and use it for other thing.

[roubert]

Could you point out how you generate this PR by tool so we can learn from that and use it for other thing.

The core of it was this:

$ clang-tidy --fix --header-filter='.' --checks='-*,modernize-redundant-void-arg'

(In the future, I intend to add information about Clang-Tidy to the ICU documentation, with instructions on how to create an LLVM compilation database and so on, including a default configuration in the ICU source code repository.)