Revert "Support proto2 default values and field presence (#716)"

This reverts commit 0b0f084.

docs/generated_code.md

@@ -134,8 +134,8 @@ foo = "";
 bar?: string;
 ```
 
-Note that all scalar fields have a zero-value in proto3 syntax, unless they are 
-marked as `optional`. Protobuf types map to ECMAScript types as follows:
+Note that all scalar fields have an intrinsic default value in proto3 syntax, unless they are marked
+as `optional`. Protobuf types map to ECMAScript types as follows:
 
 | protobuf type | ECMAScript type | default value        |
 |---------------|-----------------|----------------------|

docs/runtime_api.md

@@ -68,9 +68,10 @@ You can create an instance with the `new` keyword:
 const user = new User();
 ```
 
+
 For convenience, constructors accept an initializer object. All fields in the 
-initializer object are optional, and if not provided, the field keeps its 
-default value.
+initializer object are optional, and if not provided, the default value for the 
+field is used.
 
 ```typescript
 const user = new User({
@@ -1162,7 +1163,12 @@ let plain: PlainMessage<User> = {
 };
 ```
 
-You can convert any message to a plain message with the function [`toPlainMessage`][src-toPlainMessage].
+As such, [`PlainMessage<T>`][src-PlainMessage] can be a great fit to use 
+throughout your business logic, if that business logic is never concerned with 
+serialization, and does not need `instanceof`.
+
+Note that any `T` (assuming `T` extends `Message`) is assignable to a variable 
+of type [`PlainMessage<T>`][src-PlainMessage].
 
 
 ### AnyMessage

packages/protobuf-bench/README.md

@@ -10,5 +10,5 @@ server would usually do.
 
 | code generator      | bundle size             | minified               | compressed         |
 |---------------------|------------------------:|-----------------------:|-------------------:|
-| protobuf-es         | 98,372 b      | 41,985 b | 10,933 b |
+| protobuf-es         | 97,378 b      | 41,628 b | 10,766 b |
 | protobuf-javascript | 394,384 b  | 288,654 b | 45,122 b |