[flang] Do not create .f18.mod files for each compiled module (#85249)

The default CMake scripts had a copy operation to copy a compiled `.mod`
file to also be available with suffix `.f18.mod`. This seems no longer
needed. Also updated ModFiles.md to point to `-module-suffix`.

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Co-authored-by: Kiran Chandramohan <kiranchandramohan@gmail.com>