Remove usage of kMimeTypeDataTransferEndpoint for lacros 3/3

kMimeTypeDataTransferEndpoint was only used as a mime type
for lacros. Given lacros is gone, this is no longer needed and
it can be safely removed.

PS all its usages were removed in https://crrev.com/c/5937931
and https://crrev.com/c/5938193

Bug: 373972275, 374122148
Change-Id: I9149b3f9eb5f806679002894537835b184269b53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5944181
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Christine Hollingsworth <christinesm@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1372283}