Drop @babel/plugin-transform-classes by default (#68290)

We shouldn't include this by default in our plugins list anymore and
this should be handled by browserslist instead via `@babel/preset-env`
if browsers old enough that need this are configured. Additionally we'll
leave this configured if specific config is being provided for it to
avoid unexpected dropping.

Closes: #65540
Closes: NEXT-3598

Author: ijjk