Remove stray XtraDB files

The XtraDB storage engine is not used in MariaDB 10.2, and
it no longer exists starting with MariaDB 10.3.1.

storage/xtradb/mysql-test/storage_engine/parts/suite.pm

-package My::Suite::SE::XtraDB;
-
-@ISA = qw(My::Suite);
-
-return "Need XtraDB engine";
-
-bless { };
-

storage/xtradb/mysql-test/storage_engine/trx/suite.pm

-package My::Suite::SE::XtraDB;
-
-@ISA = qw(My::Suite);
-
-return "Need XtraDB engine";
-
-bless { };
-