make: remove unused LIBSUNDIALS

Author: WardBrian

make/libraries

@@ -81,9 +81,6 @@ $(SUNDIALS)/lib/libsundials_nvecserial.a: $(SUNDIALS_NVECSERIAL)
 
 SUNDIALS_TARGETS ?= $(SUNDIALS)/lib/libsundials_nvecserial.a $(SUNDIALS)/lib/libsundials_cvodes.a $(SUNDIALS)/lib/libsundials_idas.a $(SUNDIALS)/lib/libsundials_kinsol.a
 
-# FIXME(dl, 02/2023): remove LIBSUNDIALS variable after replacing use in Stan and CmdStan with SUNDIALS_TARGETS
-LIBSUNDIALS = $(SUNDIALS_TARGETS)
-
 STAN_SUNDIALS_HEADERS :=  $(call findfiles,$(MATH)stan,*cvodes*.hpp) $(call findfiles,$(MATH)stan,*idas*.hpp) $(call findfiles,$(MATH)stan,*kinsol*.hpp)
 $(STAN_SUNDIALS_HEADERS) : $(SUNDIALS_TARGETS)