Remove details tag from histograms metadata

Ran script that removed detail tags and moved content to summary
https://paste.googleplex.com/6555106787983360

Removed support for parsing `<details>` tag

Tested by running `merge_xml.py` and confirming no details tag present.

Similar to
https://chromium-review.googlesource.com/c/chromium/src/+/3067316
but removed tags in obsolete_histograms.xml as well

Bug: chromium:1143486
Change-Id: I84b77a2603606e15d14cf7e3d9858153528c34ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3072411


Commit-Queue: David Dinata <dinata@google.com>
Auto-Submit: David Dinata <dinata@google.com>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908688}