Revert support for static mutable variables.

Of course there's no such thing.

[SVN r52349]

tools/boostbook

@@ -1 +1 @@
-Subproject commit 41398d1414adf5cd42fe5c0eec70df6b61f4318d
+Subproject commit 34c69630306a12e56ea6214c7fba0d3480c705f6