tests/file_check: remove unused variable

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12187

tests/zfs-tests/cmd/file_check/file_check.c

@@ -40,7 +40,6 @@ main(int argc, char **argv)
 	long		i, n;
 	unsigned char	fillchar = DATA;
 	int		bigbuffersize = BIGBUFFERSIZE;
-	int64_t		read_count = 0;
 
 	/*
 	 * Validate arguments
@@ -78,8 +77,6 @@ main(int argc, char **argv)
 				exit(1);
 			}
 		}
-
-		read_count += n;
 	} while (n == bigbuffersize);
 
 	return (0);