No one(!) actually implements this hook. So simply delete it.

gdb/ChangeLog:

	* solist.h (struct target_so_ops): Delete special_symbol_handling
	hook.
	* solib.c (solib_add, reload_shared_libraries): Adjust.
	* solib-aix.c (solib_aix_special_symbol_handling): Delete
	(_initialize_solib_aix): Adjust
	* solib-darwin.c (darwin_special_symbol_handling): Delete
	(_initialize_darwin_solib): Adjust
	* solib-dsbt.c (dsbt_special_symbol_handling): Delete
	(_initialize_dsbt_solib): Adjust
	* solib-frv.c (frv_special_symbol_handling): Delete
	(_initialize_frv_solib): Adjust
	* solib-svr4.c (svr4_special_symbol_handling): Delete
	(_initialize_svr4_solib): Adjust
	* solib-target.c (solib_target_special_symbol_handling): Delete
	(_initialize_solib_target): Adjust