ARC: perf: Remove redundant initialization of variable idx

The variable idx is being initialized with a value that is never
read, it is being updated later on. The assignment is redundant and
can be removed.

Reviewed-by: Vladimir Isaev <isaev@synopsys.com>
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>