package.json: remove @babel/polyfill
package.json: remove @babel/polyfill

This package was giving a very large deprecation warning when running
npm, and it looks like nothing is actually using it anymore, so let's
remove it.

Closes #12300

cockpituous/cockpit,deryni/cockpit,mvollmer/cockpit,garrett/cockpit,deryni/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpituous/cockpit,deryni/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,garrett/cockpit,deryni/cockpit,martinpitt/cockpit,cockpituous/cockpit,cockpit-project/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,garrett/cockpit,garrett/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,deryni/cockpit,mvollmer/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,deryni/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,cockpituous/cockpit