remove non-working and non-supported $substitute feature for weapons (Mantis #2951)

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10082 387891d4-d844-0410-90c0-e4c51a9137d3
"new" shouldn't really ever fail

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10083 387891d4-d844-0410-90c0-e4c51a9137d3