Do not set unneeded  Slicer_PREREQUISITES_CONFIGURED option.

Following Slicer r23180, setting this option is not needed. It was
failed attempt to avoid the error message reported below.

The update done to SlicerConfig check a variable that is only defined
by project() command and ensure we won't get any false error.

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"To be effective, 'find_package(Slicer COMPONENTS ConfigurePrerequisites)' should "
"be placed before any calls to 'project()' or 'enable_language()'."
"Before re-configuring, make sure to clean the build directory: ${CMAKE_CURRENT_BINARY_DIR}"
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