[help wanted] add unit-test for RemoveVolume

[zhuangqh]

Ⅰ. Issue Description

Add unit-test for Volume Core's RemoveVolume method which locate on storage/volume/core.go.

Ⅱ. Describe what happened

Ⅲ. Describe what you expected to happen

Ⅳ. How to reproduce it (as minimally and precisely as possible)

Ⅴ. Anything else we need to know?

please "Baiji" developer refer to this issue in your PR for the convenience for us to count your score.

Ⅵ. Environment:

• pouch version (use pouch version):
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:

[zhuangqh]

Close via #1679