Remove unrelated change

Author: martin-frbg

Makefile.system

@@ -754,8 +754,6 @@ CCOMMON_OPT += -DF_INTERFACE_GFORT
 FCOMMON_OPT += -Wall
 # make single-threaded LAPACK calls thread-safe #1847
 FCOMMON_OPT += -frecursive
-# work around ABI changes in gfortran 9 that break calls from C code
-FCOMMON_OPT += -fno-optimize-sibling-calls
 #Don't include -lgfortran, when NO_LAPACK=1 or lsbcc
 ifneq ($(NO_LAPACK), 1)
 EXTRALIB += -lgfortran