MDEV-14606 Assertion failure on IMPORT TABLESPACE
MDEV-14606 Assertion failure on IMPORT TABLESPACE

fseg_alloc_free_page_low(): Remove a bogus and redundant assertion about
fil_space_t::purpose. The debug function fsp_space_modify_check()
is asserting something similar, but more accurately.

natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2