[DEVOPS-673] buildkite: remove CARDANO_SL_BRANCH definitions
[DEVOPS-673] buildkite: remove CARDANO_SL_BRANCH definitions

This variable is not used any more but got added back in a merge.

input-output-hk/daedalus,input-output-hk/daedalus,input-output-hk/daedalus,input-output-hk/daedalus,input-output-hk/daedalus,input-output-hk/daedalus