fix(bazel): remove cyclic dependency by removing usage of assign_external_projects_to_separate_corpora config_setting flag (#6146)

Usage of assign_external_projects_to_separate_corpora along with select statement is causing a cyclic dependency for Bazel 8. Also not usage of flag assign_external_projects_to_separate_corpora was found hence deprecating the flag. The flag was initially introduced for backward compatibility in https://source.corp.google.com/h/github/kythe/kythe/+/3966e3f7aa790a61645e6babf511f7ae21b78011.

Tested by executing bazelisk --bazelrc=$KYTHE_RELEASE_DIR/extractors.bazelrc build //src:bazel which succeeded.