Remove some unnecessary declarations and configury

Remove some unnecessary declarations and configury

configure checks for declarations of free, malloc, and realloc; but
the results are only used in a single spot: utils.c.  I think these
checks are long since obsolete, so this patch removes them.

Grepping the entire tree for these HAVE_DECL_ symbols, the only uses
of possible interest to gdb come from bfd/sysdep.h; but this is not
(nor should be) included by gdb.  (And furthermore I think the code
there is probably also obsolete.)

gdb/ChangeLog
2018-07-24  Tom Tromey  <tom@tromey.com>

	* utils.c (malloc, realloc, free): Don't declare.
	* configure, config.in: Rebuild.
	* configure.ac: Don't check for declarations of free, malloc, or
	realloc.