i18n: Merge similar translation strings for non-hierarchical taxonomy

[ramiy]

Merge two similar translation strings:

• This taxonomy is not hierarchical so you can't set a parent. - used in rest api (x2)
• Cannot set parent term, taxonomy is not hierarchical. - used in xmlrpc

See the screenshot from GlotPress and the patch merging the strings.

[SergeyBiryukov]

In 41574:

I18N: Merge two similar "Cannot set parent term" error strings.

Props ramiy.
Fixes #41637.

[netweb]

Reopening, per ​https://travis-ci.org/WordPress/wordpress-develop/builds/278799825

There was 1 failure:


1) Tests_XMLRPC_wp_editTerm::test_parent_for_nonhierarchical

Failed asserting that two strings are equal.

--- Expected

+++ Actual

@@ @@

-'This taxonomy is not hierarchical so you can't set a parent.'

+'Cannot set parent term, taxonomy is not hierarchical.'


/home/travis/build/WordPress/wordpress-develop/tests/phpunit/tests/xmlrpc/wp/editTerm.php:89

Patch 41637.2.patch​ updates the tests string to match the string added in [41574]

[SergeyBiryukov]

In 41575:

XML-RPC: Update the string in test_parent_for_nonhierarchical() to match the string added in [41574].

Props netweb.
Fixes #41637.