graphs-depends: merge redundant-dependencies elimination

Merge the redundant-dependencies elimination into the newly introduced
transitive-dependencies elimination.

This makes the code cleaner and much shorter, because:

  - the ('all',pkg) redundant dependency is in fact a transitive
    dependency, and we now have code to deal with that

  - the (pkg,'toolchain') dependency is easy enough to deal with that
    having a separate function for that is overkill

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>