Simplify Document

[schmidt-sebastian]

This basically reverts #561 since it will no longer be needed when all FieldValues expose the full Proto interface. It also brings back the old signature to the Document constructor.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (mrschmidt/rewritefieldvalue@9354294). Click here to learn what that means.
The diff coverage is 87.5%.

Flag	Coverage Δ	Complexity Δ
#FirebaseFirestore	62.22% <87.5%> (?)	2344 <1> (?)
#FirebaseFirestore_Ktx	41.17% <ø> (?)	0 <ø> (?)

Impacted Files	Coverage Δ	Complexity Δ
...firebase/firestore/model/mutation/SetMutation.java	64% <100%> (ø)	8 <0> (?)
...ogle/firebase/firestore/local/LocalSerializer.java	94.11% <100%> (ø)	25 <0> (?)
...se/firestore/model/mutation/TransformMutation.java	81.92% <100%> (ø)	22 <0> (?)
.../com/google/firebase/firestore/model/Document.java	83.78% <100%> (ø)	20 <1> (?)
...rebase/firestore/model/mutation/PatchMutation.java	82.22% <100%> (ø)	16 <0> (?)
...le/firebase/firestore/remote/RemoteSerializer.java	79.31% <50%> (ø)	147 <0> (?)

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[wilhuff]

LGTM