Test galera checks that a `GRA_x_x.log` file is created whenever
wsrep applier fails to apply some replication event. The file
contains the corresponding binlog event that failed to apply.
The test creates a new file by concatenating a pre-recorded
file containing the binlog header (see `std-data/binlog-header.log`)
and the `GRA_x_x.log` file. The test then checks that the resulting
file, containing the binlog header and the event that failed to
apply, is correctly read by `mysqlbinlog` program.
The test fails in MariaDB because the GRA_x_x.log file created
by MariaDB already contains the binlog header (see MDEV-7867).
This patch fixes/simplifies test `galera.galera_gra_log` so that
it doesn't concatenate `std-data/binlog-header.log` with the
`GRA_x_x.log` file. File `std-data/binlog-header.log` is deleted
altoghether, because not used by any other test.