Make repository fully REUSE compliant

Hi,

This repo failed reuse lint, so I fixed it by adding a missing LICENSE file and removing an unnecessary one.

Thanks,

Maxwell

reuse lint output before this change:

# MISSING LICENSES

'BSD-3-Clause' found in:
* CMakePresets.json


# UNUSED LICENSES

The following licenses are not used:
* MIT


# SUMMARY

* Bad licenses:
* Deprecated licenses:
* Licenses without file extension:
* Missing licenses: BSD-3-Clause
* Unused licenses: MIT
* Used licenses: BSD-2-Clause, CC0-1.0, GPL-2.0-only, GPL-2.0-or-later,
* GPL-3.0-only, GPL-3.0-or-later, LGPL-2.0-or-later, LGPL-2.1-or-later,
* LGPL-3.0-or-later, LicenseRef-KDE-Accepted-GPL,
* LicenseRef-Qt-Commercial-exception-1.0
* Read errors: 0
* Files with copyright information: 119 / 119
* Files with license information: 119 / 119

reuse lint output after this change:

# SUMMARY

* Bad licenses:
* Deprecated licenses:
* Licenses without file extension:
* Missing licenses:
* Unused licenses:
* Used licenses: BSD-2-Clause, BSD-3-Clause, CC0-1.0, GPL-2.0-only,
* GPL-2.0-or-later, GPL-3.0-only, GPL-3.0-or-later, LGPL-2.0-or-later,
* LGPL-2.1-or-later, LGPL-3.0-or-later, LicenseRef-KDE-Accepted-GPL,
* LicenseRef-Qt-Commercial-exception-1.0
* Read errors: 0
* Files with copyright information: 119 / 119
* Files with license information: 119 / 119

Congratulations! Your project is compliant with version 3.0 of the REUSE
Specification :-)