Patch: Remove all instance fields/methods
Patch: Remove all instance fields/methods

Nothing uses instances of Patch anymore. Hence, remove all
fields/methods and disallow the class to be instantiated. 'Patch' is
still kept as wrapper class as it contains some other classes/constants
which are still used. Moving some of those somewhere else would also
break the serialization of the diff caches, which we don't want to flush
right now just because of a cleanup.

Change-Id: I1e11f3aa58b2aeeab0eac0a1a13eda3321ed23b1

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