[Fix] Use rspamd event wrapper consistently for TCP session timer

[Fix] Use rspamd event wrapper consistently for TCP session timer

The TCP session timer was incorrectly mixing rspamd's rspamd_io_ev wrapper
with direct libev API calls (ev_timer_init/start/stop), creating inconsistent
state that could lead to resource management issues.

Fixed by using rspamd_ev_watcher_init/start/stop consistently throughout,
passing fd=-1 for pure timers without file descriptors. Also removed the
now-unused fuzzy_tcp_timer_libev_cb wrapper function.