DEV-41358: Remove the code for the old search interface.

This checkin deletes all of the code associated with the old XUI-based
search interface, including LLFloaterDirectory, LLPanelDir*, a global
variable event hack, and the associated XUI file. The new web-based
search interface, LLFloaterSearch, is the way of the future.

Deleted 19 files and 5,838 lines of code.