autotools: remove FORCE_UNSAFE_CONFIGURE from docs, commands, code (#16145)

Since #16132, we've consolidated the setting of FORCE_UNSAFE_CONFIGURE to
`autotools.py`, so we don't need to use it in packages like `coreutils`,
in our commands, or in our container recipes.

- [x] Remove FORCE_UNSAFE_CONFIGURE from packages
- [x] Remove FORCE_UNSAFE_CONFIGURE from container recipes
- [x] Remove FORCE_UNSAFE_CONFIGURE from `spack ci` command

share/spack/docker/centos-6.dockerfile

@@ -5,7 +5,6 @@ ENV DOCKERFILE_BASE=centos            \
     DOCKERFILE_DISTRO=centos          \
     DOCKERFILE_DISTRO_VERSION=6       \
     SPACK_ROOT=/opt/spack             \
-    FORCE_UNSAFE_CONFIGURE=1          \
     DEBIAN_FRONTEND=noninteractive    \
     CURRENTLY_BUILDING_DOCKER_IMAGE=1 \
     container=docker

share/spack/docker/centos-7.dockerfile

@@ -5,7 +5,6 @@ ENV DOCKERFILE_BASE=centos            \
     DOCKERFILE_DISTRO=centos          \
     DOCKERFILE_DISTRO_VERSION=7       \
     SPACK_ROOT=/opt/spack             \
-    FORCE_UNSAFE_CONFIGURE=1          \
     DEBIAN_FRONTEND=noninteractive    \
     CURRENTLY_BUILDING_DOCKER_IMAGE=1 \
     container=docker

share/spack/docker/ubuntu-1604.dockerfile

@@ -5,7 +5,6 @@ ENV DOCKERFILE_BASE=ubuntu:16.04      \
     DOCKERFILE_DISTRO=ubuntu          \
     DOCKERFILE_DISTRO_VERSION=16.04   \
     SPACK_ROOT=/opt/spack             \
-    FORCE_UNSAFE_CONFIGURE=1          \
     DEBIAN_FRONTEND=noninteractive    \
     CURRENTLY_BUILDING_DOCKER_IMAGE=1 \
     container=docker
@@ -90,4 +89,3 @@ RUN spack spec hdf5+mpi
 
 ENTRYPOINT ["/bin/bash", "/opt/spack/share/spack/docker/entrypoint.bash"]
 CMD ["docker-shell"]
-

share/spack/docker/ubuntu-1804.dockerfile

@@ -5,7 +5,6 @@ ENV DOCKERFILE_BASE=ubuntu            \
     DOCKERFILE_DISTRO=ubuntu          \
     DOCKERFILE_DISTRO_VERSION=18.04   \
     SPACK_ROOT=/opt/spack             \
-    FORCE_UNSAFE_CONFIGURE=1          \
     DEBIAN_FRONTEND=noninteractive    \
     CURRENTLY_BUILDING_DOCKER_IMAGE=1 \
     container=docker