Revert "Use upstream default ${libexecdir} now that Debian Policy allows /usr/libexec (via FHS 3.0)"

This reverts commit 7f39916226.

debian/changelog

@@ -1,7 +1,5 @@
 ostree (2018.7-1) UNRELEASED; urgency=medium
 
-  * Use upstream default ${libexecdir} now that Debian Policy allows
-    /usr/libexec (via FHS 3.0)
   * Standards-Version: 4.1.5 (no changes required)
   * New upstream release
     - d/libostree-1-1.symbols: Update