BREAK: manual.yaml: don't set any default env vars

Remove the 'MAKEFLAGS' and 'V' defaults.

Now that there is a specialised 'make' element, these make-specific
defaults don't make sense here. This element is meant to be a blank
slate for folks to build on.

_versions.py: BST_FORMAT_VERSION bumped to 18.

buildstream/_versions.py

@@ -23,7 +23,7 @@
 # This version is bumped whenever enhancements are made
 # to the `project.conf` format or the core element format.
 #
-BST_FORMAT_VERSION = 17
+BST_FORMAT_VERSION = 18
 
 
 # The base BuildStream artifact version

buildstream/plugins/elements/manual.yaml

-# No variables added for the manual element by default, set
-# this if you plan to use make, and the sources cannot handle
-# parallelization.
-#
-# variables:
-#
-#   notparallel: True
-
 # Manual build element does not provide any default
 # build commands
 config:
@@ -28,14 +20,3 @@ config:
   strip-commands:
   - |
     %{strip-binaries}
-
-# Use max-jobs CPUs for building and enable verbosity
-environment:
-  MAKEFLAGS: -j%{max-jobs}
-  V: 1
-
-# And dont consider MAKEFLAGS or V as something which may
-# effect build output.
-environment-nocache:
-- MAKEFLAGS
-- V