github: delete stranded SelectAccountCommandImpl

When `aab6fef` ("github: merge .UI.Avn and .UI in to GitHub") folded
the separate `.UI.Avalonia` and `.UI` projects back into the main
GitHub project, it deleted the sibling
{Credentials,DeviceCode,TwoFactor}CommandImpl.cs files that lived
under `src/shared/GitHub.UI.Avalonia/Commands/`. The
`SelectAccountCommandImpl.cs` that had been added two months earlier
in `483d6d3` ("github: add prompt to select accounts") was missed.

That left a lone source file under a directory belonging to a
project that no longer exists: no `.csproj`, no solution entry, no
consumer instantiates it. `GitHubAuthentication.ShowSelectAccountPromptAsync`
calls `AvaloniaUi.ShowViewAsync<SelectAccountView>` directly rather
than going through the helper-command process, so deleting the file
is a true no-op.

Remove the orphan file and its directory, completing what `aab6fef`
started. The abstract `SelectAccountCommand` HelperCommand base in
`GitHub/UI/Commands/` is left in place — it is dead code along with
its three abstract siblings, but that is a pre-existing condition
out of scope for this commit.

Assisted-by: Claude Opus 4.7
Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham