home-manager: remove unsupported -2 option

This option used to make the `home-manager` command use the `nix` tool
from Nix 2. Unfortunately the `nix` tool is a bit experimental and it
is best to await its stabilization before supporting it in Home
Manager.

home-manager/home-manager

@@ -110,22 +110,12 @@ function doBuildAttr() {
         extraArgs=("${extraArgs[@]}" "--show-trace")
     fi
 
-    # shellcheck disable=2086
-    if [[ -v USE_NIX2_COMMAND ]]; then
-        nix build \
-            -f "<home-manager/home-manager/home-manager.nix>" \
-            "${extraArgs[@]}" \
-            "${PASSTHROUGH_OPTS[@]}" \
-            --argstr confPath "$HOME_MANAGER_CONFIG" \
-            --argstr confAttr "$HOME_MANAGER_CONFIG_ATTRIBUTE"
-    else
     nix-build \
         "<home-manager/home-manager/home-manager.nix>" \
         "${extraArgs[@]}" \
         "${PASSTHROUGH_OPTS[@]}" \
         --argstr confPath "$HOME_MANAGER_CONFIG" \
         --argstr confAttr "$HOME_MANAGER_CONFIG_ATTRIBUTE"
-    fi
 }
 
 # Presents news to the user. Takes as argument the path to a "news
@@ -191,13 +181,8 @@ function doBuild() {
 
     local exitCode
 
-    if [[ -v USE_NIX2_COMMAND ]]; then
-        doBuildAttr activationPackage \
-            && exitCode=0 || exitCode=1
-    else
     doBuildAttr --attr activationPackage \
         && exitCode=0 || exitCode=1
-    fi
 
     presentNews "$newsInfo"
 
@@ -219,17 +204,10 @@ function doSwitch() {
     # before activation completes.
     generation="$WORK_DIR/generation"
 
-    if [[ -v USE_NIX2_COMMAND ]]; then
-        doBuildAttr \
-                    --out-link "$generation" \
-                    activationPackage \
-            && "$generation/activate" || exitCode=1
-    else
     doBuildAttr \
                 --out-link "$generation" \
                 --attr activationPackage \
         && "$generation/activate" || exitCode=1
-    fi
 
     presentNews "$newsInfo"
 
@@ -329,14 +307,6 @@ function buildNews() {
     local output
     output="$WORK_DIR/news-info.sh"
 
-    if [[ -v USE_NIX2_COMMAND ]]; then
-        doBuildAttr \
-                    --out-link "$output" \
-                    --quiet \
-                    --arg check false \
-                    --argstr newsReadIdsFile "$(newsReadIdsFile)" \
-                    newsInfo
-    else
     doBuildAttr \
                 --out-link "$output" \
                 --no-build-output \
@@ -345,7 +315,6 @@ function buildNews() {
                 --argstr newsReadIdsFile "$(newsReadIdsFile)" \
                 --attr newsInfo \
                 > /dev/null
-    fi
 
     echo "$output"
 }
@@ -498,9 +467,6 @@ while [[ $# -gt 0 ]]; do
         build|instantiate|edit|expire-generations|generations|help|news|packages|remove-generations|switch|uninstall)
             COMMAND="$opt"
             ;;
-        -2)
-            USE_NIX2_COMMAND=1
-            ;;
         -A)
             HOME_MANAGER_CONFIG_ATTRIBUTE="$1"
             shift