[libc++][NFC] Remove obsolete --osx-roots parameter to run-buildbot

That isn't used anymore since we now run backdeployment testing
on the target system directly instead of using pre-packaged roots.

Author: ldionne

libcxx/utils/ci/run-buildbot

@@ -28,10 +28,6 @@ ${PROGNAME} [options] <BUILDER>
 --build-dir <DIR>   The directory to use for building the library. By default,
                     this is '<llvm-root>/build/<builder>'.
 
---osx-roots <DIR>   Path to pre-downloaded macOS dylibs. By default, we download
-                    them from Green Dragon. This is only relevant at all when
-                    running back-deployment testing if one wants to override
-                    the old dylibs we use to run the tests with different ones.
 Environment variables
 CC                  The C compiler to use, this value is used by CMake. This
                     variable is optional.
@@ -66,10 +62,6 @@ while [[ $# -gt 0 ]]; do
             BUILD_DIR="${2}"
             shift; shift
             ;;
-        --osx-roots)
-            OSX_ROOTS="${2}"
-            shift; shift
-            ;;
         *)
             BUILDER="${1}"
             shift