Drop a double entry in rtl_simulation.yaml

We ended up with the Unicode fix twice because of two colliding PR
merges. We only need one copy of the -CFLAGS argument, and VCS
generates a rather strange message about an unknown argument if you
use it twice. Fortunately, it's easy to fix once you've worked out how
to get the system to print out what it's doing.

dv/uvm/core_ibex/yaml/rtl_simulation.yaml

@@ -33,7 +33,6 @@
           -timescale=1ns/10ps
           -licqueue
           -LDFLAGS '-Wl,--no-as-needed'
-          -CFLAGS '--std=c99 -fno-extended-identifiers'
           -Mdir=<tb_dir>/vcs_simv.csrc
           -o <tb_dir>/vcs_simv
           -debug_access+pp