Remove unnecessary fillstruct from devcontainer

fillstruct fills a struct literal with default values[1]
This functionality is now available in the gopls language server[2]

[1] https://github.com/davidrjenni/reftools/tree/master/cmd/fillstruct
[2] golang/go#37576 (comment)

Author: johnboyes

.devcontainer/Dockerfile

@@ -30,7 +30,6 @@ RUN apt-get update \
         honnef.co/go/tools/...@latest \
         github.com/mdempsky/gocode@latest \
         github.com/sqs/goreturns@latest \
-        github.com/davidrjenni/reftools/cmd/fillstruct@latest \
         github.com/uudashr/gopkgs/v2/cmd/gopkgs@latest  \
         github.com/ramya-rao-a/go-outline@latest  \
         github.com/acroca/go-symbols@latest  \