[signin] Histogram ChromePrimaryAccountStateOnWebSignout is obsolete

Bug: 975524
Change-Id: Iafd0d275778777146af7c1cd47389facb127af31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1778873
Commit-Queue: David Roger <droger@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Auto-Submit: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693072}