Description:
It is noticed that there are a lot of redundant inc files which
are not being used by any of the MTR tests. Also, there are several
inc files which are included in the test but are not needed
anymore.

Fix:

Below inc files are not being used by any test and hence removed:

1.  include/check_key_reads.inc
2.  include/check_key_req.inc
3.  include/commandline_option_test.inc
4.  include/config_file_option_test.inc
5.  include/explain.inc
6.  include/finish_option_test.inc
7.  include/have_binlog_checksum_off.inc
8.  include/have_dynamic_loading.inc
9.  include/have_shm.inc
10. include/init_option_test.inc
11. include/not_blackhole.inc
12. include/show_partition_checksums.inc
13. include/show_partition_contents.inc
14. include/test_outfile.inc

Below are a few inc files which are used in MTR tests but are no more needed.
Hence these inc files along with their references from all MTR tests have been
removed.

1. have_symlink.inc
2. have_perfschema.inc
3. have_innodb.inc

Note: include/not_valgrind_server.inc is not being used by any MTR test but
still not removed as this was introduced as part of Bug#22447330

Reviewed by:
Parveez Baig <parveez.baig@oracle.com>
Deepa Dixit <deepa.dixit@oracle.com>