default-distrovars.inc: Remove unused variable 'LIMIT_BUILT_LOCALES'

This variable is a left-over from early glibc recipes.
It is not referenced anymore in oe-core, and its functionality
has been replaced with 'GLIBC_GENERATE_LOCALES'.

Also remove the reference to it in local.conf.sample.extended.

Signed-off-by: Jacob Kroon <jacob.kroon@mikrodidakt.se>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>