Remove exit_inferior_num_silent

Remove exit_inferior_num_silent

The sole caller of exit_inferior_num_silent was getting the inferior's
number to then use the number to look up the inferior again.  I think
it's better to simply not have exit_inferior_num_silent; any potential
callers that only have the inferior's number should probably be
converted to pass the inferior itself around instead.

Tested by the buildbot.

gdb/ChangeLog
2018-07-03  Tom Tromey  <tom@tromey.com>

	* infrun.c (follow_exec): Use exit_inferior_silent.
	* inferior.c (exit_inferior_num_silent): Remove.
	* inferior.h (exit_inferior_num_silent): Don't declare.