fix-obj-vars has the undesired side effect of breaking -cflags
-objs and -libs variables in the toplevel Makefile.objs.  The
variables in the toplevel Makefile.objs do not need any fix,
so fix-obj-vars need not do anything.

Since we are touching it, remove the now unnecessary $(if)
in the callers.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>