[TabModel] Simplify TabGroupObserver#didRemoveTabGroup signature

Remove the unused `oldRootId` parameter (hardcoded to
`Tab.INVALID_TAB_ID` across all callers) and drop `@Nullable` from
`oldTabGroupId` in `TabGroupObserver#didRemoveTabGroup`.

`oldTabGroupId` was only nullable in the legacy
`TabGroupModelFilterImpl` when changing a group's `rootId` while reusing
its `Token` during a merge. In `TabCollectionTabModelImpl`, reused
tokens are removed from `candidateTabGroupIds` in
`mergeListOfTabsToGroup` and never trigger `didRemoveTabGroup`, so all
three emission sites (`closingTabGroupIds`, `candidateTabGroupIds`, and
`moveTabInternal`) always pass a non-null `Token`.

Also add the missing `DidRemoveTabGroupReason.PIN` constant to the
`@IntDef` and drop the unreachable `else if (isMergingIntoGroup)` branch
in `moveTabInternal`: `isMergingIntoGroup` requires `newTabGroupId !=
null`, whereas the enclosing block requires `newTabGroupId == null`
(`MERGE` removals are emitted in `mergeListOfTabsToGroup` after the move
loop).

Bug: 517544602
Change-Id: I65e2e2bf04f239e310591b85a5e2db58775fcfe0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8466467
Auto-Submit: Gazal Agarwal <agazal@google.com>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Gazal Agarwal <agazal@google.com>
Cr-Commit-Position: refs/heads/main@{#1704873}