Connect-AzAccount -MaxContextPopulation not working when called with existing contexts

## Description

`Connect-AzAccount -MaxContextPopulation` not working when called with existing contexts.

## Steps to reproduce

First delete `~/.azure`.

```powershell
> Connect-AzAccount
# Returned 1 item in total

> Get-AzContext -ListAvailable
# Returned 25 items in total

> Connect-AzAccount -MaxContextPopulation 100
# Returned 1 item in total

> Get-AzContext -ListAvailable
# Still returned 25 items in total
```

## Environment data

```powershell
> $PSVersionTable

Name Value
---- -----
PSVersion 7.1.0
PSEdition Core
GitCommitId 7.1.0
OS Microsoft Windows 10.0.19042
Platform Win32NT
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
WSManStackVersion 3.0
```

## Module versions

```powershell
> Get-Module -ListAvailable

Directory: C:\Users\xxx\Documents\PowerShell\Modules

ModuleType Version PreRelease Name PSEdition ExportedCommands
---------- ------- ---------- ---- --------- ----------------
Script 5.6.0 Az Core,Desk
Script 2.2.6 Az.Accounts Core,Desk {Disable-AzDataCollection, Disable-AzCo…
```