Remove cleanups from linux-tdep.c

Remove cleanups from linux-tdep.c

This removes some cleanups from linux-tdep.c, replacing them with
def_vector or byte_vector as appropriate.

gdb/ChangeLog
2017-11-04  Tom Tromey  <tom@tromey.com>

	* linux-tdep.c (linux_core_info_proc_mappings): Use
	gdb::def_vector.
	(linux_get_siginfo_data): Return gdb::byte_vector.  Remove
	"size" argument.
	(linux_corefile_thread): Update.
	(linux_make_corefile_notes): Remove unused variable.