Fix build error caused by unused variable in OSX

[ChungHsuanChen]

Remove unused local variable in fvtest which will cause build error
In omrtimeTest.cpp

In si.cpp

Changes

• omrtimeTest.cpp, replace for loop with while loop as variable 'j' isn't used
• si.cpp, remove l that not use as any purpose

[github-actions]

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[babsingh]

https://github.com/eclipse/omr/blob/master/CONTRIBUTING.md#commit-guidelines

The following commit message requirement is not met: The body should be wrapped at 72 characters, where reasonable.

[knn-k]

Jenkins build amac

[knn-k]

LGTM.
The test failure on x86-64 macOS is not caused by this PR. See #7181.