doc: remove references to downstream

Having references to downstream no-longer make sense now that we've
unsubtree'd.

src/univalue/sources.mk

@@ -1,12 +1,8 @@
-# - All variables are namespaced with UNIVALUE_ to avoid colliding with
-#     downstream makefiles.
 # - All Variables ending in _HEADERS or _SOURCES confuse automake, so the
 #     _INT postfix is applied.
 # - Convenience variables, for example a UNIVALUE_TEST_DIR should not be used
 #     as they interfere with automatic dependency generation
-# - The %reldir% is the relative path from the Makefile.am. This allows
-#   downstreams to use these variables without having to manually account for
-#   the path change.
+# - The %reldir% is the relative path from the Makefile.am.
 
 UNIVALUE_INCLUDE_DIR_INT = %reldir%/include