Fix newer collations for proper space padding in MyRocks

This is fixed in Facebook MyRocks 8.0 and will need to be ported to MyRocks with Percona Server 8.0.
This will require index version bump to maintain backwards compatibility data previously stored in Percona Server 8.0.
The FB implementation is at https://github.com/facebook/mysql-5.6/commit/c5bd81632b55

1. Revert code changes related to MY_STRXFRM_NOPAD_WITH_SPACE at
https://github.com/Percona/percona-server/commit/309003670166ebd084ed0f63d5a49a534006a682
2. Revert "cs->mbmaxlen" at https://github.com/Percona/percona-server/commit/2452a628610f11c92f3a1f0d5cb27fc8ab8f445a