Remove SCons building system

While SCons building system was serving us really good for ages it's no longer
having much attention by the developers and started to become quite a difficult
task to maintain.

What's even worse -- there started to be quite serious divergence between SCons
and CMake which was only accumulating over the releases now. The fact that none
of the active developers are really using SCons and that our main studio is also
using CMake spotting bugs in the SCons builds became quite a difficult task and
we aren't always spotting them in time.

Meanwhile CMake became really mature building system which is available on every
platform we support and arguably it's also easier and more robust to use.

This commit includes:

- Removal of actual SCons building system
- Removal of SCons git submodule
- Removal of documentation which is stored in the sources and covers SCons
- Tweaks to the buildbot master to stop using SCons submodule
  (this change requires deploying to the server)
- Tweaks to the install dependencies script to skip installing or mentioning
  SCons building system
- Tweaks to various helper scripts to avoid mention of SCons folders/files
  as well

Reviewers: mont29, dingto, dfelinto, lukastoenne, lukasstockner97, brecht, Severin, merwin, aligorith, psy-fi, campbellbarton, juicyfruit

Reviewed By: campbellbarton, juicyfruit

Differential Revision: https://developer.blender.org/D1680

source/creator/CMakeLists.txt

@@ -524,7 +524,6 @@ if(UNIX AND NOT APPLE)
 			unset(_pypath_real)
 
 			# Copy the systems python into the install directory
-			# Scons copy in tools/Blender.py
 			# install(CODE "message(\"copying a subset of the systems python...\")")
 			install(
 				DIRECTORY ${PYTHON_LIBPATH}/python${PYTHON_VERSION}