Remove the PERFVIEW define

[sharwell]

DRAFT The PERFVIEW preprocessor define is no longer required. For all code which is not shared (using file links in the build) between both the PerfView and other projects, its value can be inlined to true (for the PerfView project) or false (for other projects). If at the end of this no more references to PERFVIEW exist, it can also be removed from the constants defined in PerfView.cs.

@vancem Can you review the above for accuracy before I implement it?

[github-actions]

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