Removed unnecessary lock to call acmp_process_quick in Pm::evaluate

- This was introduced in commit 119a6fc & 7d786b3 because of a potential
  issue reported in #1573.
- The ACMP tree structure is initialized when the operator is
  initialized.
- During transaction execution the ACMP tree structure is only 'read'
  while traversing the tree (in acmp_process_quick) so this is safe for
  use in a multi-threaded environment.

src/operators/pm.h

@@ -53,12 +53,6 @@ class Pm : public Operator {
 
  protected:
     ACMP *m_p;
-
-#ifdef MODSEC_MUTEX_ON_PM
-
- private:
-    std::mutex m_mutex;
-#endif
 };