spack.yaml: Drop oppulance (transient dependency of pynn-brainscales)

We can drop oppulance from `spack.yaml` as it is no top-level
dependency anymore.

spack.yaml

@@ -30,7 +30,6 @@ spack:
     - tvb-framework ^binutils+ld+gold
     - pynn-brainscales@2.0-rc1 ^log4cxx@0.10.0 ^googletest@1.11.0:+gmock
     - py-neo
-    - oppulance@2.0-rc2
     #- py-cerebstats
     #- py-cerebunit
     - py-cerebrus