monitor: Remove unused lambda capture

Fix clang-tidy error:
```
../monitor/system.cpp:421:24: error: lambda capture 'this' is not used [-Werror,-Wunused-lambda-capture]
```

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I933eed9d132c4599ccefa250b5bd6e9b1774f4c6
diff --git a/monitor/system.cpp b/monitor/system.cpp
index 9620c09..12f2c03 100644
--- a/monitor/system.cpp
+++ b/monitor/system.cpp
@@ -418,7 +418,7 @@
 
         // Cancel any in-progress power off actions
         std::for_each(_powerOffRules.begin(), _powerOffRules.end(),
-                      [this](auto& rule) { rule->cancel(); });
+                      [](auto& rule) { rule->cancel(); });
     }
 }