Fail to execute command "Azure Functions: Delete Function App..." from command palette

[v-chenzha]

OS: Win10
Build Version: Azure Function v1.9.1-alpha.0 + Azure Resources v0.5.7-alpha.0

Repro Steps:

1. F1 -> Azure Functions: Delete Function App... -> Select a subscription -> Select a resource.
2. Check whether succeeds to execute command "Azure Functions: Delete Function App..." directly.

Expect:
Succeed to execute command "Azure Functions: Delete Function App..." directly.

Actual:
Continue to perform the next step and fail to execute command "Azure Functions: Delete Function App...".

More Info:
This issue also reproduces when executing command "Azure Functions: Configure Deployment Source..." from command palette.

[alexweininger]

Note: to fix this, we need to stop passing an expected child context value when we call pick app resource.

[alexweininger]

Fixed in #3507