thunderbird: don't force-enable the sandbox

This should fix build on aarch64, as there's no sandboxing possible.
https://bugzilla.mozilla.org/show_bug.cgi?id=1294331
As the code defaults to enabling it where it's possible, let's drop the option.

pkgs/applications/networking/mailreaders/thunderbird/default.nix

@@ -72,7 +72,6 @@ in stdenv.mkDerivation rec {
       "--enable-system-sqlite"
       #"--enable-system-cairo"
       "--enable-startup-notification"
-      "--enable-content-sandbox"            # available since 26.0, but not much info available
       "--disable-crashreporter"
       "--disable-tests"
       "--disable-necko-wifi" # maybe we want to enable this at some point