Remove unused `using` directives in ax_platform_node_auralinux.h

These are not used, so remove them to reduce any confusion in codesearch

Change-Id: Ie340780a91b263fa6554ea49c5a4ec0335b6a0e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2734454


Auto-Submit: Daniel Libby <dlibby@microsoft.com>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#859738}