Remove <no-bounds> on trait objects

Printing <no-bounds> on trait objects comes from a time when trait
objects had a non-empty default bounds set. As they no longer have any
default bounds, printing <no-bounds> is just noise.
auto merge of #14057 : kballard/rust/remove_no-bounds, r=sfackler

Printing <no-bounds> on trait objects comes from a time when trait
objects had a non-empty default bounds set. As they no longer have any
default bounds, printing <no-bounds> is just noise.