Revert "Forward the return key to the omniboxKeyboardDelegate"

This reverts commit 93bb62f6533c59aad8545d2d986a3d1c65cfa0ef.

Reason for revert:
LUCI Bisection has identified this change as the cause of a test failure. See the analysis: https://ci.chromium.org/ui/p/chromium/bisection/test-analysis/b/6282128460349440

Sample build with failed test: https://ci.chromium.org/b/8708486844634219489
Affected test(s):
[ninja://ios/chrome/test/earl_grey2:ios_chrome_ui_eg2tests_module/QRScannerViewControllerTestCase/testReceivingQRScannerSearchQueryResultAndEditingTheQuery](https://ci.chromium.org/ui/test/chromium/ninja:%2F%2Fios%2Fchrome%2Ftest%2Fearl_grey2:ios_chrome_ui_eg2tests_module%2FQRScannerViewControllerTestCase%2FtestReceivingQRScannerSearchQueryResultAndEditingTheQuery?q=VHash%3Ae65a2513aae56189)

If this is a false positive, please report it at http://b.corp.google.com/createIssue?component=1199205&description=Analysis%3A+https%3A%2F%2Fci.chromium.org%2Fui%2Fp%2Fchromium%2Fbisection%2Ftest-analysis%2Fb%2F6282128460349440&format=PLAIN&priority=P3&title=Wrongly+blamed+https%3A%2F%2Fchromium-review.googlesource.com%2Fc%2Fchromium%2Fsrc%2F%2B%2F6773725&type=BUG

Original change's description:
> Forward the return key to the omniboxKeyboardDelegate
>
> This CL fixes a bug where the copied-link suggestion was not navigated
> to when hitting the return key.
> Prior to this CL, the return key was not triggering any call to
> OmniboxTextFieldIOS nor its delegate. This CL adds the same explicit
> handling as for arrow keys.
>
> Fixed: 40836724
> Change-Id: I1f444e9e7ec6c14468064102c02a9047b15f4c99
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6773725
> Reviewed-by: Christian Xu <christianxu@chromium.org>
> Auto-Submit: Louis Romero <lpromero@google.com>
> Commit-Queue: Louis Romero <lpromero@google.com>
> Cr-Commit-Position: refs/heads/main@{#1491240}
>

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I04296bfe0e53a480853528b5a0aff92c8af125e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6782769
Reviewed-by: Christian Xu <christianxu@chromium.org>
Reviewed-by: Filipa Senra <fsenra@google.com>
Reviewed-by: Louis Romero <lpromero@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Filipa Senra <fsenra@google.com>
Cr-Commit-Position: refs/heads/main@{#1491327}