[Synapse] Add a Remove-AzSynapseSqlPoolRestorePoint cmdlet

[zesluo]

Guidelines
Do your cmdlets comply with the design guidelines outlined in the PowerShell Design Guidelines document?

Y
Service Release Details
Is this an Embargoed Preview, A Public Preview, or a General Release?

Public Preview
What is the expected service release date?

2020-11
Contact Information
Main developer contacts (emails + github aliases)

zesluo@microsoft.com;
PM contact (email + github alias)

Matthew.Hicks@microsoft.com matt1883
Other people who should attend a design review (email)

dongwwa@microsoft.com
High Level Scenarios
Describe how your feature is intended to be used by customers.

Add a Remove-AzSynapseSqlPoolRestorePoint cmdlets
Remove-AzSynapseSqlPoolRestorePoint allow data engineers to remove a restorepoint in Synapse Sql pool

ParentObject piping scenarios are supported in these cmdlets.
PS C:> $pool = Get-AzSynapseSqlPool -WorkspaceName ContosoWorkspace -Name ContosoSqlPool
PS C:> $pool | Remove-AzSynapseSqlPoolRestorePoint -Name ContosoSqlPoolRestorePointCreationTime
Syntax changes
This should include PowerShell-help style syntax descriptions of all new and changed cmdlets, similar to the syntax portion of PowerShell help (or markdown help), for example:

New Cmdlet
Link to cmdlet help markdown file generated by platyPS or sample syntax:

Remove-AzSynapseSqlPoolRestorePoint
PS C:> Remove-AzSynapseSqlPoolRestorePoint -WorkspaceName -SqlPoolName -RestorePointCreationDate $RestorePointCreationDate

Additional information
Link to the OpenAPI (swagger) spec

{ LINK HERE }