unlink keyword for :dir argument of ld and include-book

Example Forms:
(delete-include-book-dir :smith)
 ; Remove association of directory with :smith for include-book.

General Form:
(delete-include-book-dir kwd)
where kwd is a keywordp. The effect of this event is to modify the meaning of the :dir keyword argument of include-book and ld as indicated by the examples above, namely by removing association of any directory with the indicated keyword for purposes of the include-book (and ld) :dir argument. Normally one would instead use add-include-book-dir to associate a new directory with that keyword; see add-include-book-dir.

Note: This is an event! It does not print the usual event summary but nevertheless changes the ACL2 logical world and is so recorded.

This macro is local to any books and encapsulate events in which it occurs; see add-include-book-dir for a discussion of this aspect of both macros.