Drop shelljs package from tfjs-node.

[nkreeger]

There is a security problem with shelljs and that package has not been published in 7 months.

It looks like we do this only for tensorboard tests - we can do these actions with existing Node.js APIs already:

src/tensorboard_test.ts:24:const shelljs = require('shelljs');
src/tensorboard_test.ts:37:      shelljs.rm('-rf', tmpLogDir);
src/tensorboard_test.ts:133:    shelljs.mkdir('-p', tmpLogDir);
src/tensorboard_test.ts:156:      shelljs.rm('-rf', tmpLogDir);