rt: Remove circular_buffer

Author: brson

mk/rt.mk

@@ -63,7 +63,6 @@ RUNTIME_CXXS_$(1) := \
               rt/rust_log.cpp \
               rt/rust_gc_metadata.cpp \
               rt/rust_util.cpp \
-              rt/circular_buffer.cpp \
               rt/isaac/randport.cpp \
               rt/miniz.cpp \
               rt/rust_kernel.cpp \