Remove dead const.

[bcsgh]

https://github.com/aws/aws-sdk-cpp/issues/2906

Description of changes: Remove an unused const that is a duplicate of another const in another namespace.

Check all that applies:

• [x] Did a review by yourself.
• [ ] Added proper tests to cover this PR. -- No tests: if it doesn't break the build it should be correct.
• [x] Checked if this PR is a breaking (APIs have been changed) change.
• [x] Checked if this PR will not introduce cross-platform inconsistent behavior. -- This should (I think) remove cross-platform inconsistent behavior
• [x] Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• [x] Linux
• [ ] Windows
• [ ] Android
• [ ] MacOS
• [ ] IOS
• [ ] Other Platforms

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