[ GRAPH ] Remove unused function and add doxygen note

[ GRAPH ] Remove unused function and add doxygen note

In neural network class, there is fucntions which should be moved to
graph.
In this PR, remove member functions which is not used any more and add
doxygen comment in graph header.

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

Signed-off-by: jijoong.moon <jijoong.moon@samsung.com>