[IME] Remove input pane flag.

This is enabled for quite some time now.

Bug: 40565153
Change-Id: I6f3df7c62371dd9ff32bad3c4be8465a87849edd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402258


Commit-Queue: Anupam Snigdha <snianu@microsoft.com>
Reviewed-by: Sanket Joshi <sajos@microsoft.com>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279966}