Fix leak in splay-tree

Fix leak in splay-tree

Philippe Waroquiers noticed a memory leak in gdb, which he tracked
down to a bug in splay-tree.  splay_tree_remove does not call the
`delete_key' function when it removes the old node; but it should.

I looked at every splay tree in GCC and there is only one that passes
a non-NULL delete function -- the one in lto.c.  That file does not
call splay_tree_remove.  So, I think this is safe to check in.

I re-ran the LTO tests to double check.

libiberty/
	* splay-tree.c (splay_tree_remove): Delete the key if necessary.