Remove/Reserve PerStepNamespaceMetrics fields from GlobalGetDataRequest

[JayajP]

Some notes:

• This is effectively a safe rollback of pr/29482.
• We will send this data through DFE instead of WMW.
• User Workers are currently not populating these fields.
• Matching changes to the internal proto have already been made (see cl/595227885 for reference.

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