Fixes #16444: Remove nusoap from 1.3 release

In 1.2.x releases we have deprecated the usage of nusoap in favor of using the
PHP SOAP extension due to speed, memory, and some encoding issues.
In 1.3 release, we should drop the dependency on nusoap completely.

This change includes the following:
- Removal of the library.
- Removed all references to it.
- Added a check section for webservice to make sure that the extension is enabled and nusoap folder doesn't exist.
- Added a check in mantisconnect.php to exit with an error message if the php extension doesn't exist.