Delete deprecated NSIS installer references

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Describe the bug
During configuration time, CMake is trying to detect if NSIS is installed. If NSIS is not installed on the system, the following is output on the console:

-- Could NOT find NSIS (missing: NSIS_MAKE)

This message is confusing to users because is looks like a warnings message when NSIS is never used by ShellAnything.

To Reproduce
Steps to reproduce the behavior:

1. Configure the project.
2. CMake will try to detect if NSIS is installed.
3. The message Could NOT find NSIS (missing: NSIS_MAKE) is displayed on the console.

Expected behavior
NSIS is not required anymore for the project. The custom detection script FindNSIS.cmake is not required anymore. It should have been deleted when #101 was implemented. The find_package(NSIS) from CMakeLists.txt line 8 shall also be removed.

Screenshots
N/A

Environment
N/A

Additional context
N/A