[selection] Remove feature flags for mouse selection in shadow dom

Remove feature flag MouseFocusFlatTreeParent, which was to fix a bug
about mouse focus not falling back to correct slotted content. This was
shipped in 133 and is safe to remove in 136.

Remove feature flag SelectionOnShadowDOMWithDelegatesFocus, which was
to fix a bug about mouse focus not allowing text selection in
shadow dom when delegatesFocus is set to true. This was shipped in 133
and is safe to remove in 136.

Change-Id: I2c9d4499f63c4cc1fbe12fae907cd0d8708080ac
Bug: 41420461
Bug: 40622041
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6377261
Commit-Queue: Di Zhang <dizhangg@chromium.org>
Reviewed-by: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1435796}