initdb: replace check_icu_locale() with default_icu_locale().

initdb: replace check_icu_locale() with default_icu_locale().

The extra checks done in check_icu_locale() are not necessary. An
existing comment already pointed out that the checks would be done
during post-bootstrap initialization, when the locale is opened by the
backend. This was a mistake in commit 27b62377b4.

This commit creates a simpler function default_icu_locale() to just
return the locale of the default collator.

Discussion: https://postgr.es/m/04182066-7655-344a-b8b7-040b1b2490fb%40enterprisedb.com
Reviewed-by: Peter Eisentraut