tests: fix e2e image build

tests/Dockerfile

@@ -1,5 +1,5 @@
 # stick to non-alpine so we can reuse build from travis host (avoid libc compat issues)
-FROM golang:1.13
+FROM golang:1.14
 
 ARG DEBIAN_FRONTEND=noninteractive
 
@@ -8,11 +8,8 @@ RUN apt update \
  && rm -rf /var/lib/apt/lists/*
 RUN go get -d golang.zx2c4.com/wireguard \
  && cd /go/src/golang.zx2c4.com/wireguard \
- && rm donotuseon_linux.go \
  && make install
 
-ENV WG_I_PREFER_BUGGY_USERSPACE_TO_POLISHED_KMOD=1
-
 COPY entrypoint.sh /
 
 WORKDIR /app