Manual element has defaults that only belong in make element

Summary

There are defaults in the 'manual' element, that are only relevant to the 'make' element.

These are:

  MAKEFLAGS: -j%{max-jobs}
  V: 1

We should take advantage of the API breaks allowed in v1.4 to remove these.

Steps to reproduce

See the docs

What is the current bug behavior?

Environment vars are present.

What is the expected correct behavior?

There should be no defaults in the 'manual' element, it should be a blank slate.

• BuildStream version affected: /milestone %BuildStream_v1.4

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