Remove a check in `MadviseAwayAlternateSignalStack()`.

We're testing and benchmarking ART on devices with old
kernels which may not support `MADV_FREE`, so the
`madvise()` call may fail in these test environments.
It should succeed on devices with Android 12+.

Test: Rely on TreeHugger.
Bug: 38383823
Change-Id: I633cc8dce1be013a31cd5fb756b498d5043b2550