Get rid of corefile.c:close_exec_file

Get rid of corefile.c:close_exec_file

The body of this function has been commented out since Jul 1999, and
thus seems unnecessary. While at it, remove some commented out code
that seems to be related to the function being deleted.

gdb/ChangeLog:

	* corefile.c (close_exec_file): Delete.
	(reopen_exec_file): Remove commented out code that seems related
	to close_exec_file, which is being deleted here.
	* inferior.h (close_exec_file): Delete.
	* fork-child.c (fork_inferior): Remove call to fork_inferior.