[trivial] modify flatten_realizer script for consistent codes

[trivial] modify flatten_realizer script for consistent codes

modify "flatten_realizer.cpp" script for consistent codes of
"flatten_realizer.cpp" and "activation_realizer.cpp".

"flatten_realizer" and "activation realizer" contain exactly same
implementation but written in slightly different ways.

It's a very trivial matter, but I thought it would be better to
write the same implementation in the same code, so I modified it.

**Self evaluation:**
1. Build test: [x]Passed []Failed []Skipped
2. Run test: [x]Passed []Failed []Skipped

Signed-off-by: Seungbaek Hong <sb92.hong@samsung.com>