Remove some cleanups from solib-svr4.c

Remove some cleanups from solib-svr4.c

This removes a few cleanups from solib-svr4.c in a straightforward
way.

gdb/ChangeLog
2018-03-30  Tom Tromey  <tom@tromey.com>

	* solib-svr4.c (lm_info_read): Use gdb::byte_vector.  Return
	std::unique_ptr.
	(svr4_keep_data_in_core): Update.
	(svr4_read_so_list): Update.