Change breakpoint::filter to be a unique_xmalloc_ptr

Change breakpoint::filter to be a unique_xmalloc_ptr

This changes breakpoint::filter to be a unique_xmalloc_ptr, removing
an explicit xfree, as well as a use of a "release" method.

gdb/ChangeLog
2019-08-06  Tom Tromey  <tromey@adacore.com>

	* breakpoint.c (init_breakpoint_sal): Update.
	(breakpoint): Update.
	* breakpoint.h (struct breakpoint) <filter>: Now a
	unique_xmalloc_ptr.