sim: cleanup unused SIM_EXTRA_CFLAGS

We want to eventually delete this, so at least drop the empty ones.

sim/iq2000/Makefile.in

diff --git a/sim/iq2000/Makefile.in b/sim/iq2000/Makefile.in
index 757ed28..e7e0c71 100644
--- a/sim/iq2000/Makefile.in
+++ b/sim/iq2000/Makefile.in
@@ -31,8 +31,6 @@
 	$(CGEN_INCLUDE_DEPS) \
 	arch.h cpuall.h $(srcdir)/../../opcodes/iq2000-desc.h eng.h
 
-SIM_EXTRA_CFLAGS =
-
 ALL_CPU_CFLAGS = -DHAVE_CPU_IQ2000BF -DHAVE_CPU_IQ10BF
 
 SIM_EXTRA_CLEAN = iq2000-clean

sim/lm32/Makefile.in

diff --git a/sim/lm32/Makefile.in b/sim/lm32/Makefile.in
index db15bef..0519159 100644
--- a/sim/lm32/Makefile.in
+++ b/sim/lm32/Makefile.in
@@ -19,9 +19,6 @@
 SIM_EXTRA_DEPS = $(CGEN_INCLUDE_DEPS) $(srcdir)/../../opcodes/lm32-desc.h \
 	eng.h
 
-# List of flags to always pass to $(CC).
-#SIM_EXTRA_CFLAGS =
-
 SIM_EXTRA_CLEAN = lm32-clean
 
 ## COMMON_POST_CONFIG_FRAG

sim/or1k/Makefile.in

diff --git a/sim/or1k/Makefile.in b/sim/or1k/Makefile.in
index f288b02..e845637 100644
--- a/sim/or1k/Makefile.in
+++ b/sim/or1k/Makefile.in
@@ -49,8 +49,6 @@
 	cpuall.h \
 	decode.h
 
-SIM_EXTRA_CFLAGS =
-
 SIM_EXTRA_LIBS = -lm
 
 ## COMMON_POST_CONFIG_FRAG