Revert "[SelectMenu] Allow the UA provided shadow DOM to be replaced via attachShadow."

This reverts commit f0c3efe4e884df8226b3a3a2dff98dec86597f6d.
Some conflicts were resolved manually.

Per the OpenUI resolution at [1], <selectmenu> will not support
attachShadow(). This CL updates the implementation to match
that resolution by reverting the earlier change that added
this functionality.

[1] openui/open-ui#528

Bug: 1121840
Change-Id: Ie7b606b327c07e5e3f477e0c0982e2158515ef07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3692009
Reviewed-by: Mason Freed <[email protected]>
Commit-Queue: Dan Clark <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1012007}

Author: dandclark