Drop try_run macro from CMake's own build

Merged Brad King requested to merge brad.king/cmake:drop-try_run-macro into master

Since commit 9199f7c6 we've abused an undocumented debugging feature to override the builtin try_run command in CMake's own build with a wrapper macro. However, we've also long discouraged use of this feature by other projects. The purpose of the original change is outdated and of limited use anyway, so just drop it.

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