configure: remove needless if leg

It was pointed out in review of the previous patch that the if leg
isn't needed as the for loop will not enter on an empty $device_archs.

Fixes: d1d5e9eefd ("configure: allow the selection of alternate config in the build")
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210720232703.10650-5-alex.bennee@linaro.org>

configure

@@ -5103,12 +5103,10 @@ if test "$skip_meson" = no; then
   echo "[properties]" >> $cross
 
   # unroll any custom device configs
-  if test -n "$device_archs"; then
-      for a in $device_archs; do
-          eval "c=\$devices_${a}"
-          echo "${a}-softmmu = '$c'" >> $cross
-      done
-  fi
+  for a in $device_archs; do
+      eval "c=\$devices_${a}"
+      echo "${a}-softmmu = '$c'" >> $cross
+  done
 
   test -z "$cxx" && echo "link_language = 'c'" >> $cross
   echo "[built-in options]" >> $cross