Remove proxy info from Dockerfile.ubuntu18.04

[ashahba]

If users want to build behind corporate proxy, they can always build it this way:

$ docker build \
    --build-arg HTTP_PROXY=${HTTP_PROXY} \
    --build-arg HTTPS_PROXY=${HTTPS_PROXY} \
    --build-arg NO_PROXY=${NO_PROXY} \
    --build-arg http_proxy=${http_proxy} \
    --build-arg https_proxy=${https_proxy} \
    --build-arg no_proxy=${no_proxy} \
    -f Dockerfile.ubuntu18.04 .

[ashahba]

@avijit-nervana
@crlishka

[ashahba]

/assign @kanvi-nervana