Eliminate async_annotation_suffix

Eliminate async_annotation_suffix

The comments and existence of this global are a bit of misleading
obfuscation, since this is only ever used to print the prompt
annotation, and never changes.  Just hardcode "prompt" where
necessary, as done for most other annotations.

gdb/ChangeLog:
2016-03-09  Pedro Alves  <palves@redhat.com>

	* event-top.c (async_annotation_suffix): Delete.
	(top_level_prompt, command_line_handler): Don't use
	'async_annotation_suffix' and simplify.
	* event-top.h (async_annotation_suffix): Delete declaration.
	(init_main): Remove reference to 'async_annotation_suffix'.