Remove -c flag from expand command

[mittz]

This change has been tested with the following test cases. This is similar to Remove the -c flag for create and makes -c backwards compatible.

Case 1: ghpc expand help

This command doesn't show the -c flag in the list of flags as it's deprecated.

$ ./ghpc expand --help
Updates the YAML Config in the same way as create, but without writing the blueprint.

Usage:
  ghpc expand [flags]

Flags:
  -h, --help         help for expand
  -o, --out string   Output file for the expanded yaml. (default "expanded.yaml")

Case 2: ghpc expand without -c flag

Simple success.

$ ./ghpc expand examples/hpc-cluster-small.yaml 
Expanded config created successfully, saved as expanded.yaml.

$ ls expanded.yaml 
expanded.yaml

Case 3: ghpc expand with -c flag (Deprecation)

Success with the deprecation message.

$ ./ghpc expand -c examples/hpc-cluster-small.yaml
Flag --config has been deprecated, please see the command usage for more details.
Expanded config created successfully, saved as expanded.yaml.

$ ls expanded.yaml 
expanded.yaml

Submission Checklist:

• Have you installed and run this change against pre-commit? pre-commit install
• Are all tests passing? make tests
• If applicable, have you written additional unit tests to cover this
  change?
• Is unit test coverage still above 80%?
• Have you updated any application documentation such as READMEs and user
  guides?
• Have you followed the guidelines in our Contributing document?