NaCl GN build: Remove some unneeded #defines

NaCl GN build: Remove some unneeded #defines

This syncs the GN build with Gyp.

The per-OS #defines were removed from Gyp here:
https://codereview.chromium.org/788193003

The per-arch #defines were removed from Gyp here:
https://codereview.chromium.org/902903002

BUG=440012
TEST=build

Committed: https://crrev.com/9ae8d7875f0ad6b8511940131640d920749982ec
Cr-Commit-Position: refs/heads/master@{#349537}

[Mark Seaborn, 2015-09-17 19:43:03]

mseaborn@chromium.org changed reviewers:
+ dpranke@chromium.org

[Dirk Pranke, 2015-09-17 20:07:53]

lgtm

[Mark Seaborn, 2015-09-17 22:13:23]

The CQ bit was checked by mseaborn@chromium.org

[commit-bot: I haz the power, 2015-09-17 22:13:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1351933003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1351933003/1

[commit-bot: I haz the power, 2015-09-17 23:29:54]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-17 23:30:27]

Patchset 1 (id:??) landed as
https://crrev.com/9ae8d7875f0ad6b8511940131640d920749982ec
Cr-Commit-Position: refs/heads/master@{#349537}