Remove Microsoft.Extensions.Configuration.DockerSecrets from metapackage

[Tratcher]

This package is marked as noship/exclude, but it's included in the full FullMetaPackagePackageReference. This causes restore errors in VS:

Error	NU1102	Unable to find package Microsoft.Extensions.Configuration.DockerSecrets with version (>= 2.0.0-preview2-25722)
  - Found 2 version(s) in NuGet [ Nearest version: 2.0.0-preview1-final ]
  - Found 1 version(s) in CliFallbackFolder [ Nearest version: 2.0.0-preview2-25661 ]
  - Found 0 version(s) in SignOff

@HaoK @JunTaoLuo @muratg

Preview2 blocking

[JunTaoLuo]

We need #19