phosphor-ipmi-flash: Code Health Cleanup

Removed the following warning.
- using decl '*' is unused
- 'push_back' is called inside a loop; consider pre-allocating the
    container capacity before the loop
- function '*' defined in a header file; function
    definitions in header files can lead to ODR violations

Change-Id: I44e4b0f8056a853fb45b690394be1a8ebec45b20
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/bmc/test/triggerable_mock.hpp b/bmc/test/triggerable_mock.hpp
index 0db90ee..4ae4463 100644
--- a/bmc/test/triggerable_mock.hpp
+++ b/bmc/test/triggerable_mock.hpp
@@ -22,9 +22,6 @@
     using TriggerableActionInterface::cb;
 };
 
-std::unique_ptr<TriggerableActionInterface> CreateTriggerMock()
-{
-    return std::make_unique<TriggerMock>();
-}
+std::unique_ptr<TriggerableActionInterface> CreateTriggerMock();
 
 } // namespace ipmi_flash