Cleanup value_fetch_lazy's comment and return value

Cleanup value_fetch_lazy's comment and return value

The comment for value_fetch_lazy seems outdated. It says that it's only
called from the value_contents and value_contents_all (macros!), which
is not true.  Also, the return value seems useless now, despite what the
comment says.

gdb/ChangeLog:

	* value.c (value_fetch_lazy): Update comment, change return
	value to void.
	* value.h (value_fetch_lazy): Change return value to void.