Remove plenum and node specific dependencies.

[pSchlarb]

Since node and plenum fixed the dependencies settings described in
hyperledger/indy-node#1799
and fixed in
hyperledger/indy-plenum#1635

The specific dependencies (eg. ursa, rocksdb) should be removed in the Dockerfiles to not cause issues when they are updated in node/plenum.

[Echsecutor]

I will try to just remove the dependencies from the docker file for RC6. If the build goes through, I will create a PR.

[pSchlarb]

Since it is fixed in #127, i will close this issue.