Remove "dll-symbols", "add-shared-symbol-files" and assf" commands.

Remove "dll-symbols", "add-shared-symbol-files" and assf" commands.

This patch removes a set of commands that have been deprecated for
a while, and which we agreed to remove after the GDB 7.8 release.

gdb/ChangeLog:

	* windows-nat.c (safe_symbol_file_add_stub)
	(safe_symbol_file_add_cleanup, safe_symbol_file_add)
	(dll_symbol_command): Delete.
	(_initialize_windows_nat): Delete local variable "c".
	Remove "dll-symbols", "add-shared-symbol-files" and assf"
	commands.

Tested by rebuilding GDB on x86-windows.