Remove "linux_multi_process" global

Remove "linux_multi_process" global

The "linux_multi_process" is initialized but never modified.  I
discussed this with Pedro on irc, and he said that, while it was
useful when developing this feature, it is now no longer needed.  So,
this removes it.

gdb/ChangeLog
2020-07-18  Tom Tromey  <tom@tromey.com>

	* linux-nat.c (linux_multi_process): Remove.
	(linux_nat_target::supports_multi_process): Return true.