Cleanup locale tools:

• Simplify the source dir specification, and update README appropriately
• Drop the LC (doonly) processing, it's broken, and even if fixed, not really useful
• Don't remove the target directories while installing new data as it removes Makefile.depend which we don't manage; only rm the files we are going to add/replace/delete instead
• Restrict adding bsd.endian.mk to colldef and ctypedef Makefiles, it's not needed in other (text-only) categories
• GC unused scripts; they don't seem to be particularly helpful standalone as well

Reviewed by: bapt
Approved by: kib (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D17858