What does ChopMD refer to in the default Go ECDSA package?
Let $n$ be the digest length of a hash function $\operatorname{MD}^f:\{0,1\}^*\to\{0,1\}^n$. Now quoting the paper you linked:


  chopMD. For $0\leq s&lt;n$ we define $\operatorname{chop}_s(x)=x_R$ where
  $x=x_L\parallel x_R$ and $\left|x_L\right|=s$. In this paper we fix
  $0&lt;s&lt;n$ and define
  $\operatorname{chopMD}^f(M)=\operatorname{chop}_s(\operatorname{MD}^f(M))$.


So the idea is basically "take the value, run it through the specified hash function and discard ("chop off") the first $s$ bits", which is a pretty intuitive and standard ways to get a small-domain hash function from a large-domain hash function.

So TL;DR: It appears the package runs the given data fields through SHA-512 and then only returns the last 256 bit of the resulting hash.