Remove duplicate option from documentation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320258 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CommandGuide/llvm-dwarfdump.rst

@@ -35,11 +35,6 @@ OPTIONS
             the :option:`--debug-info`, :option:`--find`,
             and :option:`--name` options.
 
-.. option:: --diff
-
-            Emit the output in a diff-friendly way by omitting offsets and
-            addresses.
-
 .. option:: -f <name>, --find=<name>
 
             Search for the exact text <name> in the accelerator tables