Remove legacy go.mod and go.sum files from _build directory
The build process left a go.mod and go.sum file in the _build directory in the past. This causes problems these days when attempting to embed the auxiliary binaries into Gitaly binary as they are considered to be in a different module due to this. Remove the legacy files prior to building Gitaly.
Closes #4381 (closed)
Edited by Sami Hiltunen