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/image_mock.hpp b/bmc/test/image_mock.hpp
index 9999f58..8d84dbe 100644
--- a/bmc/test/image_mock.hpp
+++ b/bmc/test/image_mock.hpp
@@ -23,9 +23,6 @@
     MOCK_METHOD(int, getSize, (), (override));
 };
 
-std::unique_ptr<ImageHandlerMock> CreateImageMock()
-{
-    return std::make_unique<ImageHandlerMock>();
-}
+std::unique_ptr<ImageHandlerMock> CreateImageMock();
 
 } // namespace ipmi_flash