MDEV-18096 The server would crash when has configs rpl_semi_sync_master_enabled = OFF rpl_semi_sync_master_wait_no_slave = OFF

The patch fixes a fired assert in the semisync master module. The assert
caught attempt to switch semisync off (per rpl_semi_sync_master_wait_no_slave = OFF)
when it was not even initialized (per rpl_semi_sync_master_enabled = OFF).
The switching-off execution branch is relocated under one that executes
enable_master() first.

A minor cleaup is done to remove the int return from two functions that
did not return anything but an error which could not happen in the functions.