Change StorageMap's remove() to use sha256(key, self.field_id()) instead of sha256(key, self.slot())

[bitzoic]

Description

Currently, the StorageMap remove() function uses the slot to compute the key to remove. When inserting, field_id is used. This has been updated. Tests that remove from a nested StorageMap have also been added.

Closes #6128

Checklist

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