Remove a check in `MadviseAwayAlternateSignalStack()`, again.

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: m
Bug: 38383823
Bug: 215310343
Change-Id: I2d2a7603da4e42b119a1606ab379b496bcdadfdb