Remove support to convert deprecated service flag

This code is there to support migration of a flag that is not used
since M-48. According to the histogram, 100% of the user have been
converted on Chrome OS and Android (the two platforms that still
support child accounts).

Bug: 960028
Change-Id: I2ca06df6876c96b6cd8407cab429e32e4751090d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609899
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659629}
3 files changed