Remove browse-kill-ring-highlight-inserted-item-style.

Allow browse-kill-ring-highlight-inserted-item-style to take the value
nil to indicate no highlighting, and remove
browse-kill-ring-highlight-inserted-item.

ChangeLog:

	Remove `browse-kill-ring-highlight-inserted-item' and instead
	allow `browse-kill-ring-highlight-inserted-item-style' to take the
	value `nil' meaning no highlighting.

	The default for `browse-kill-ring-highlight-inserted-item-style'
	is now nil for no highlighting, this matches the old initial value
	for `browse-kill-ring-highlight-inserted-item' which was nil by
	default.

	Given that I've removed an existing configuration variable I've
	bumped the version number.