Remove old migration guides

Cf. <php/doc-en#157>.

Author: cmb69

manual.xml.in

@@ -482,12 +482,6 @@
   &appendices.migration71;
   &appendices.migration70;
   &appendices.migration56;
-  &appendices.migration55;
-  &appendices.migration54;
-  &appendices.migration53;
-  &appendices.migration52;
-  &appendices.migration51;
-  &appendices.migration5;
   &appendices.debugger;
   &appendices.configure.index;
   &appendices.ini;