Remove stale documentation from InternalsManual.rst

The DuplicatesAllowedWhileMerging was removed a while ago,
but the documentation remained.

llvm-svn: 337835

Author: Erich Keane

clang/docs/InternalsManual.rst

@@ -1891,11 +1891,6 @@ attribute classes. For instance, ``ParsedAttr`` is the shared
 parsed attribute kind, but ARMInterruptAttr and MSP430InterruptAttr are the
 semantic attributes generated.
 
-By default, when declarations are merging attributes, an attribute will not be
-duplicated. However, if an attribute can be duplicated during this merging
-stage, set ``DuplicatesAllowedWhileMerging`` to ``1``, and the attribute will
-be merged.
-
 By default, attribute arguments are parsed in an evaluated context. If the
 arguments for an attribute should be parsed in an unevaluated context (akin to
 the way the argument to a ``sizeof`` expression is parsed), set