[backend] Delete mock graal file for deletion history

One of the edge cases for deletion history test

Signed-off-by: inishchith <inishchith@gmail.com>

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
#	deleted:    backends/graal.py
#
