Remove unused `last_swipe_timestamp_` variable

It's assigned to but never read from.

BUG=none
TEST=unit tests

Change-Id: I376c6c439212a320e80ce1ad24cc685ac0f7d26c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/gestures/+/2136225
Tested-by: Harry Cutts <hcutts@chromium.org>
Auto-Submit: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Sean O'Brien <seobrien@chromium.org>
Commit-Queue: Harry Cutts <hcutts@chromium.org>
2 files changed