build: Remove useless code in opal_mca.m4

Remove an environment variable that probably did something at one
point, but is dead code now.  Git history is hard to follow due
to some reverts in the SVN days.

Signed-off-by: Brian Barrett <bbarrett@amazon.com>

config/opal_mca.m4

@@ -705,15 +705,12 @@ AC_DEFUN([MCA_COMPONENT_COMPILE_MODE],[
         [str="STATIC_COMPONENT=\$STATIC_$2_$3"
          eval $str])
 
-    shared_mode_override=static
-
     # Setup for either shared or static
     if test "$STATIC_FRAMEWORK" = "1" || \
        test "$STATIC_COMPONENT" = "1" || \
        test "$STATIC_all" = "1" ; then
         $4="static"
-    elif test "$shared_mode_override" = "dso" || \
-         test "$SHARED_FRAMEWORK" = "1" || \
+    elif test "$SHARED_FRAMEWORK" = "1" || \
          test "$SHARED_COMPONENT" = "1" || \
          test "$DSO_all" = "1"; then
         $4="dso"