Removed "Pa" prefix where it is not necessary

[petrochuk]

This PR solves issue of "Pa" repeating three times. Microsoft.PowerPlatform.PowerApps.Persistence.PaYaml.Serialization.PaYamlSerializer

[joem-msft]

What is the issue you're trying to solve here? Please do not change anything in the PaYaml folders unless there is an actual feature that needs to be changed.

The PaYaml prefix in type names are by design.

• The need for a type prefix is to provide unambiguous type names in code when multiple namespaces are added via using statements. This is the same issue we had to address the other day.
  e.g. this assembly now has two YamlSerializer classes:
  Microsoft.PowerPlatform.PowerApps.Persistence.PaYaml.Serialization.YamlSerializer
  and
  Microsoft.PowerPlatform.PowerApps.Persistence.Yaml.YamlSerializer
• The name PaYaml is designed to correlate with most of these files have the file extension .pa.yaml.
• The PaYaml in the namespace was intended to allow for the current OM and the new OM to live in the same assembly. It is a pragmatic design decision that allows for both OMs to be side-by-side at this time of transition. Eventually, the PaYaml namespace will likely be removed, renamed, and likely moved to a different assembly. Yet, the class names will not need to change since their use of 'PaYaml' is providing a different purpose.

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