Change POM license name to 'MIT' and remove license URL for standard compliance

[eygraber]

See cashapp/licensee#137

MIT is the valid SPDX identifier, and specifying the URL breaks the Licensee plugin.

[kizitonwose]

Thanks for contributing. I'm in favour of using the correct SPDX identifier (MIT), but removing the URL does not seem right, especially because most libraries have that property set. I understand Licensee is moving in the direction where they match the name first in the future, so the URL will no longer be an issue then. Would it be worth waiting for that instead? We could just merge the SPDX identifier change for now.