.murdock: drop bors CI_BASE_BRANCH logic

.murdock

@@ -192,17 +192,6 @@ is_merge_queue_build() {
     startswith "gh-readonly-queue/" "${CI_BUILD_BRANCH}"
 }
 
-# Set CI_BASE_COMMIT in case of a merge queue build.
-# This assumes that merge queue always adds a single merge commit (possibly itself
-# including multiple merges for PRs) on the base branch.
-# That git command outputs the hash of the second merge commit (the one before
-# the topmost merge), which should be the base branch HEAD.
-# (CI_BASE_COMMIT is used by `can_fast_ci_run.py` to only build changes.)
-if test -z "${CI_BASE_COMMIT}" && is_merge_queue_build; then
-    previous_merge="$(git log --merges --max-count 1 --skip 1 --pretty=format:%H)"
-    export CI_BASE_COMMIT="${previous_merge}"
-fi
-
 # fullbuild logic
 # non-full-builds are those where can_fast_ci_run might reduce the build
 # automatically