Bug 1551726 - remove --enable-content-sandbox from old configure options; r=dmajor

The option was removed entirely several months ago, and this bit was missed.

Differential Revision: https://phabricator.services.mozilla.com/D46296

--HG--
extra : moz-landing-system : lando

build/moz.configure/old.configure

@@ -184,7 +184,6 @@ def old_configure_options(*options):
 @old_configure_options(
     '--cache-file',
     '--datadir',
-    '--enable-content-sandbox',
     '--enable-cookies',
     '--enable-cpp-rtti',
     '--enable-crashreporter',