It only handles the realloc constraint (preserving low elements) by
serendipity, and we don't actually rely on that behavior anyway.
Meanwhile the calls may do extra copying that gets immediately clobbered
by the callers.

Cherry-pick from libvpx:
3063c3760 Remove vpx_realloc()

Change-Id: I8dfa89e4a81084b084889c27bd272fdf85184e8d