[remoting] Remove caller_task_runner from DesktopSessionProxy ctor.

This parameter is not needed - the class can instead use
SequencedTaskRunner::GetCurrentDefault().

Change-Id: I1f2b5c31def53b8e2b611f39ccddc93924a858e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5408672


Auto-Submit: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280753}