Revert "Add the ability to source RBE related scripts from envsetup.sh"

This reverts commit 9aa9e47b.

Reason for revert: DroidMonitor-triggered revert due to breakage https://android-build.googleplex.com/builds/quarterdeck?branch=git_rvc-qpr-dev-plus-aosp&target=ndk&lkgb=7546337&lkbb=7546521&fkbb=7546426, bug https://buganizer.corp.google.com/issues/193566521

Fix: 193566521
Change-Id: I7196a00006499f729baa86a252b0df6535c6aeaa