licensing: remove non-standard license.txt in favor of COPYING

The new COPYING file does not directly copy the GPLv2 text, since this
license does not apply to some parts of the repository. Instead, it
documents our usage of SPDX tags and where to find license information.

COPYING

@@ -0,0 +1,10 @@
+Most of Dolphin's original source code is licensed under GPLv2+. Part of the
+source code distributed in this repository is derived from other projects and
+may have stronger or weaker licensing requirement. Overall, in aggregate, the
+entirety of this repository is compatible with the GPLv3 license.
+
+Per-file licensing information is provided via SPDX tags [1]. The licenses
+referenced in these tags can be found in the LICENSES/ directory within this
+source repository.
+
+[1] https://spdx.dev/