delete active fixes when unloading fix style plugin

src/plugin.cpp

@@ -236,6 +236,10 @@ namespace LAMMPS_NS
             lmp->force->create_pair("none",0);
         }
       }
+    } else if (pstyle == "fix") {
+      for (int ifix = lmp->modify->find_fix_by_style(name);
+           ifix >= 0; ifix = lmp->modify->find_fix_by_style(name))
+        lmp->modify->delete_fix(ifix);
     }
 
     // if reference count is down to zero, close DSO handle.