[contrib][pzstd] Fix build issue with gcc-5

[contrib][pzstd] Fix build issue with gcc-5

gcc-5 didn't like the l-value overload for defaulted operator=. There is
no reason it needs to be l-value overloaded, so just remove it.

I'm not sure why the build broke for @mckaygerhard in Issue #2811, since
this code hasn't changed since it was added. But, there is no harm in
fixing it.

Fixes issue #2811.