Remove unused Babel transformations (#6811)

The `stage-0` Babel preset has been replaced with the specific Babel
plugins that we depend upon. We don't use most of `stage-0`, so this
allowed us to remove many unnecessary transformations. We had to remove
this preset soon anyway, because all of the stage presets are
deprecated in Babel 7.

The `stage-0` preset consisted of these plugins:
```
"transform-do-expressions"
"transform-function-bind"
"transform-class-constructor-call"
"transform-export-extensions"
"transform-class-properties"
"transform-decorators"
"syntax-dynamic-import"
"syntax-trailing-function-commas"
"transform-async-generator-functions"
"transform-async-to-generator"
"transform-exponentiation-operator"
"transform-object-rest-spread"
```

Of that list, only 'transform-class-properties', 'transform-object-
rest-spread', and 'transform-async-to-generator' were being used.

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