DEP: Remove scons related files and code.

This removes files and code supporting scons builds. After this change
numpy will only support builds using distutils or bento. The removal of
scons has been discussed on the list several times and a decision has been
made that scons support is no longer needed. This was originally discussed
for numpy 1.7 and because the distutils and bento methods are still
available we are skipping the usual deprecation period.
DOC: Document removal of SCons support in doc/release/1.8-notes.rst.
BUG: Need to keep SCons support in cpucaps and cpuid.

These tools are not directly related to numpy builds.
BUG: Need to keep pointer to using scons in HOWTO_RELEASE.rst.txt.
DEP: Remove more references to scons related files.

These references were in MANIFEST.in, doc/summarize.py, release.sh, and
numpy/testing/noseclasses.py and involved scons files that have been
removed, mostly setupscons.py and setupsconsegg.py that were in the top
level.
Merge pull request #2914 from charris/remove-scons-support

Remove scons support