Remove storageClass from global variables section

[Cyb3rDudu]

Storage class has to be declared under persistence otherwise the default storageClass is taken

What this PR does / why we need it:

The storageClass is documented at the wrong place. If declared under global the chart creates the pv with the default storageClass

Pre-submission checklist:

• [x] Did you explain what problem does this PR solve? Or what new features have been added?
• [x] Have you updated the readme?
• [x] Is this PR backward compatible? If it is not backward compatible, please discuss open a ticket first