Move some code later in backtrace_command_1

Move some code later in backtrace_command_1

PR backtrace/15584 notes that some code in backtrace_command_1 is not
useful when frame filters are in use.  This patch moves this code into
the no-frame-filters "if".  This also removes the unused local
"trailing_level", which I noticed while moving the code around.

gdb/ChangeLog
2018-03-26  Tom Tromey  <tom@tromey.com>

	PR backtrace/15584:
	* stack.c (backtrace_command_1): Move some code into no-filters
	"if".