[posenet demo]Revert @babel/core to 7.6.4

[haoyunfeix]

yarn && yarn build Met error below when @babel/core version upgrade to 7.13.12

$ yarn build
yarn run v1.22.5
$ cross-env NODE_ENV=production parcel build index.html --public-url ./
×  F:\workspace\test3\tfjs-models\posenet\demo\coco.js: function __clone() {
    var node2 = new Node();
    for (var key in this) {
      // Do not clone comments tha...<omitted>... } could not be cloned.
    var node2 = new Node();
    for (var key in this) {
      // Do not clone comments tha...<omitted>... } could not be cloned.
    at Object.serialize (v8.js:256:7)
    at _default (F:\workspace\test3\tfjs-models\posenet\demo\node_modules\@babel\core\lib\transformation\util\clone-deep.js:16:30)
    at normalizeFile (F:\workspace\test3\tfjs-models\posenet\demo\node_modules\@babel\core\lib\transformation\normalize-file.js:52:36)
    at normalizeFile.next (<anonymous>)
    at run (F:\workspace\test3\tfjs-models\posenet\demo\node_modules\@babel\core\lib\transformation\index.js:31:50)
    at run.next (<anonymous>)
    at Function.<anonymous> (F:\workspace\test3\tfjs-models\posenet\demo\node_modules\@babel\core\lib\transform-ast.js:20:41)
    at Generator.next (<anonymous>)
    at evaluateSync (F:\workspace\test3\tfjs-models\posenet\demo\node_modules\gensync\index.js:251:28)
    at Function.sync (F:\workspace\test3\tfjs-models\posenet\demo\node_modules\gensync\index.js:89:14)

Revert to 7.6.4 will fix this problem.

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This change is 

[jenperson]

Thank you for your contribution! This PR is being closed because the package has been deprecated.