Fix testDeleteCategoryReassignsIssues

PHPUnit was throwing an error, due to attempting to delete the category
created for the test twice (once as part of the test itself, and again
during tearDown).

1) Mantis\tests\soap\CategoryTest::testDeleteCategoryReassignsIssues
SoapFault: Error Type: APPLICATION ERROR 0001503,
Error Description: Category "soaptest_source_20260830_121111" not found
for project "PHPUnit".

Solved by not adding the category to categoryNamesToDelete.

Issue 0032470