ASP: remove code to exit if running from live build

Since commit 8dd945ef the ASP dpkg hooks are installed in the end of the build,
making this code useless.

Refs: #14594

config/chroot_local-includes/usr/local/sbin/tails-additional-software

@@ -32,12 +32,6 @@ APT_ARCHIVES_DIR = "/var/cache/apt/archives"
 APT_LISTS_DIR = "/var/lib/apt/lists"
 
 
-def _exit_if_in_live_build():
-    """Exits with success if running inside live-build."""
-    if "SOURCE_DATE_EPOCH" in os.environ:
-        sys.exit(0)
-
-
 def _launch_apt_get(specific_args):
     """Launch apt-get with given arguments.
 
@@ -395,8 +389,6 @@ def show_system_log():
 
 def apt_hook_pre():
     """Subcommand to handle Dpkg::Pre-Install-Pkgs."""
-    _exit_if_in_live_build()
-
     syslog.syslog("Saving package changes")
 
     apt_cache = apt.cache.Cache()
@@ -453,8 +445,6 @@ def apt_hook_post():
     Retrieve the list of packages saved by apt_hook_pre, filter packages not
     interesting and pass the resulting list to the appropriate method.
     """
-    _exit_if_in_live_build()
-
     syslog.syslog("Examining package changes")
 
     with open(ASP_STATE_PACKAGES) as f: