sql: Remove Sqlite.SizeKB.* histograms.

The histograms were added in https://crrev.com/15499003 and have become
unused.

Bug: 953143
Change-Id: I32fc9eabdefe09e9a5837c3bcf4594187025f3ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1568628
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Chris Mumford <cmumford@google.com>
Reviewed-by: Steven Holte <holte@chromium.org>
Auto-Submit: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651953}