Remove key press count buffer

This was used to transmit key presses recorded by UserInputMonitor,
which was removed in the previous CL.

For IPC security reviewer: This is only removing IPC so it's likely
not security sensitive, see audio_stream_factory.mojom.

Bug: 377854424
Change-Id: I649dc59ff393c7124ed8359ff819390595f29354
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5999482
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Reviewed-by: Bryant Chandler <bryantchandler@chromium.org>
Commit-Queue: Fredrik Hernqvist <fhernqvist@google.com>
Reviewed-by: Antonio Sartori <antoniosartori@chromium.org>
Reviewed-by: Evan Liu <evliu@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1384906}