Refactor: delete Tensor::mat2vec()

Refactor: delete Tensor::mat2vec()

This method is initially proposed to flatten and clone mat to vector.

However, `Tensor::getData()` can cover most of it's purpose, it seems
no longer in need.

**Changes proposed in this PR:**
- Delete `Tensor::mat2vec()`.

See also:
https://github.com/nnstreamer/nntrainer/pull/149#discussion_r439172191

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

Signed-off-by: Jihoon Lee <jhoon.it.lee@samsung.com>