Remove unused MantidKernel/System.h import

[peterfpeterson]

As part of #38457 this removes System.h from all .cpp files. When it is needed, it is already in the corresponding .h file. The randomly added empty lines are to avoid cppcheck suppression issuse.

Refs #38457

To test:

This is a simple refactor. The builds should all pass and this does not require release notes.

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