gdbserver/server: make some functions void

The 'handle_v_attach', 'handle_v_run', and 'handle_v_kill' functions'
return values are unused.  They indicate error/success result by
putting packets.  Make the functions void.

Tested by rebuilding.

gdbserver/ChangeLog:
2021-05-06  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* server.cc (handle_v_attach)
	(handle_v_run)
	(handle_v_kill): Make void.