MDEV-9884: Existing /var/lib/mysql/.sst directory (with contents) causes SST to fail with xtrabackup-v2

[Fix taken from https://github.com/percona/percona-xtradb-
cluster/commit/b3ee75949ed82b88f355ca2e26431350cc1c89ac]

During SST, the receiver node creates .sst directory under
datadir to process/prepare the received data and removes it
at the end of the process. In case of error, this directory,
however, was not removed, which later caused subsequent SSTs
to fail. Fixed by removing this directory at the beginning
of SST if it existed.