DataItem Callback Conversion - Fin

This CL is the last of a series to convert DataItem from the deprecated
base::Callback (and Bind) to [Once|Repeating]Callback. See the relation
chain of CLs for the complete change.

Now that all methods in DataItem have been converted to the appropriate
modern types, we remove the "non-once" aliases and rename the "once"
ones back to the original name.

Bug: 1152268
Change-Id: Idc0ec68ff3d14d2557225207804f73e95976e079
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595935
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838401}