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Now modifiers are tracked in XkbTracker, so remove the cache from
exo::Keyboard.
Instead, it is cached in WaylandKeyboardDelegate for re-sending
purpose.

BUG=1123705
TEST=Ran exo_unittests locally.

Change-Id: I2800f507a030a938d2e0d58163e2e5005f1de299
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426663Reviewed-by: Jun Mukai <mukai@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810019}