cmake: dedupe expressions into local vars in cmake_uninstall.in.cmake

[vszakats]

Also: use IN LISTS syntax.

Follow-up to 27e2a47

[Copilot]

Pull request overview

This PR refactors the CMake uninstall script template (CMake/cmake_uninstall.in.cmake) to reduce repeated expressions and use more idiomatic CMake list iteration, improving readability while preserving behavior.

Changes:

• Introduces a local _manifest variable for the install_manifest.txt path and reuses it for existence checks and reading.
• Switches foreach(_file ...) to foreach(_file IN LISTS _files) after converting the manifest content into a semicolon-separated list.
• Introduces a _target local variable for $ENV{DESTDIR}${_file} to deduplicate repeated path expressions and simplify log/remove statements.

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