[Bazel] Remove explicit relative_to_caller_repository

This is the default and the argument is deprecated. The documentation
indicating it's the default is only since 4.0
(https://docs.bazel.build/versions/4.0.0/skylark/lib/Label.html#Label),
but looking at the code it has been the default since its introduction
in https://bazel.googlesource.com/bazel/+/6f15335dea with Bazel 0.2.1
(https://bazel.googlesource.com/bazel/+show/0.2.1/CHANGELOG.md)

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D106411