libc/locale: don't expose nonexistant __collate_load_error

The private symbol __collate_load_error was removed in 2015 in commit
2a6abeebef96. While it had previously been accidently removed in 2011
(commit 3c87aa1d3dc1a) and restored in 2012 (commit bb4317bf3c99) I
think it's time to write it off after four major releases without it.

Fixes: 2a6abeebef96 The collate functions within libc have...

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D44182