[module.reach] Remove impossible private-module-fragment case

Author: opensdh

source/modules.tex

@@ -1005,9 +1005,7 @@
 appears in a translation unit that is
 reachable from $P$,
 and
-either does not appear within a \grammarterm{private-module-fragment}
-or appears in a \grammarterm{private-module-fragment}
-of the module containing $P$.
+does not appear within a \grammarterm{private-module-fragment}.
 \end{itemize}
 \begin{note}
 Whether a declaration is exported has no bearing on whether it is reachable.