Revert "Try enabling VM_ASSERT in --jit CIs"

This reverts commit e9e3b65d83.

We were not ready for it
https://github.com/ruby/ruby/runs/2715306375?check_suite_focus=true

.github/workflows/mjit.yml

@@ -38,7 +38,7 @@ jobs:
       - run: ./autogen.sh
         working-directory: src
       - name: Run configure
-        run: ../src/configure -C --disable-install-doc cppflags=-DVM_CHECK_MODE
+        run: ../src/configure -C --disable-install-doc
       - run: make $JOBS incs
       - run: make $JOBS
       - run: sudo make $JOBS -s install