The "tree/polymer_leaves" benchmark has no benchmark tests, nor do we install
Polymer anywhere. Polymer was previously installed through `bower`, but since
we removed bower, there is no easy way to run this benchmark with Polymer.
Considering that there are no benchmark tests, nor we have an easy way to
install/vendor Polymer, we should just remove this benchmark app.
This is also based on the assumption that we want to remove the
Polymer benchmarks anyway: see: 8a05199fb9be5ef91ec8094a3c6abc0a65eb6bcd
PR Close#28568
7115e7c427