Unify vm benchmark prefixes to vm_ (#3028)

The vm1_ prefix and vm2_ had had special meaning until
820ad9cb1d and
12068aa4e9. AFAIK there's no special
meaning in vm3_ prefix.

As they have confused people (like "In `benchmark` what is difference
between `vm1_`, `vm2_` and `vm3_`"), I'd like to remove the obsoleted
prefix as we obviated that two years ago.

benchmark/vm_mutex.yml

@@ -0,0 +1,8 @@
+prelude: |
+  require 'thread'
+
+  m = Thread::Mutex.new
+benchmark:
+  vm_mutex: |
+    m.synchronize{}
+loop_count: 6000000