Removed extra apt-get update in apt install command

When a command like "sudo su -c "apt-get update && apt-get install" was
executed via subprocess.popen, the parent python process was suspended when the first
part of the command completed. We don't need the update anymore (it's done by the user
before running securedrop-admin setup) so let's just simplify things and run something
like "sudo apt-get install"

admin/bootstrap.py

@@ -154,13 +154,7 @@ def install_apt_dependencies(args: argparse.Namespace) -> None:
         " which was set on Tails login screen"
     )
 
-    apt_command = [
-        "sudo",
-        "su",
-        "-c",
-        f"apt-get update && \
-                   apt-get -q -o=Dpkg::Use-Pty=0 install -y {APT_DEPENDENCIES_STR}",
-    ]
+    apt_command = f"sudo apt-get -q -o=Dpkg::Use-Pty=0 install -y {APT_DEPENDENCIES_STR}".split(" ")
 
     try:
         # Print command results in real-time, to keep Admin apprised