Revert "Add @autoreleasepool to ios/chrome/app/chrome_exe_main.mm."

This reverts commit ef344143929225c301200cc7687272a3490833fa.

Reason for revert: DCHECK @ n base::AtExitManager::RegisterCallback(void (*)(void*), void*) at base/at_exit.cc:52

Original change's description:
> Add @autoreleasepool to ios/chrome/app/chrome_exe_main.mm.
> 
> This CL adds @autoreleasepool to ios/chrome/app/chrome_exe_main.mm, and
> serves as a parent CL of https://codereview.chromium.org/2887413002/
> so that ios/chrome/app:main can be converted to ARC.
> 
> Bug: 624363
> Change-Id: I434a40bc8b6a4482d03c4be7e642a218aed61b94
> Reviewed-on: https://chromium-review.googlesource.com/534874
> Reviewed-by: Mark Cogan <marq@chromium.org>
> Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
> Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#479722}

TBR=marq@chromium.org,sdefresne@chromium.org,stkhapugin@chromium.org,liaoyuke@chromium.org

Change-Id: I394073d7c4fefa4393c81f60c8932e37dca50a61
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 624363
Reviewed-on: https://chromium-review.googlesource.com/537552
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#479789}