[CMake] Remove overriding CMAKE_OSX_SYSROOT

[guitargeek]

The internet says that newer CMake versions got more reliable in figuring out the SDK path. So setting CMAKE_OSX_SYSROOT ourselves is maybe not needed anymore.

Follows up on #19218.

[guitargeek]

I don't understand what is going on at this point. PR #18318, which introduced this code, said it was to fix build errors with CMake 4. However, the two macOS platforms that use CMake 4.0.3 (mac15 and mac-beta) already succeeded building. @devajithvs, can you maybe elaborate on why #18318 was needed in the first place? As I said already in #19218 (comment), after studying the CMake source code, I don't see the reason why things should have failed with CMake 4 to begin with.

[github-actions]

Test Results

    21 files      21 suites   3d 13h 32m 13s ⏱️
 3 557 tests  3 556 ✅ 0 💤 1 ❌
72 950 runs  72 949 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 7946e41.

[devajithvs]

This was needed because, when CMake 4 on macos-15-4 first became available, it failed to set the CMAKE_OSX_SYSROOT variable. As a result, ROOT would fail to compile since the correct modulemap could not be found, as discussed here: https://root-forum.cern.ch/t/issue-with-macos-15-4-xcode-16-3/63327/2 - this was an emergency fix back then.

[guitargeek]

Thanks for giving the context! Do we still need it now? CMake 4.0 probably went to not setting CMAKE_OSX_SYSROOT probably for a good reason, so maybe we should not reproduce the previous 3.x behavior

[devajithvs]

Yes, I agree. If the build doesn't fail anymore. It's nice to revert to the previous behavior.