Delete generated BuildTypeAttr in module file to not force the build type to app consumer

[DevHugo]

Fix #416, #274.

Due to commit e7f105f the module file generated by the plugin gradle-maven-publish-plugin contains com.android.build.api.attributes.BuildTypeAttr attribute. This attribute force consumer of the library to use only the build types specified by the attribute: "debug" and "release".
See this medium article, if you want to have more details on the issue.

Reproduction case is to create an app with a specific buildTypes (not debug and release) and try to compile. Example on ReproProjectShowkase with command ./gradlew clean assemblePaid --rerun-tasks --no-configuration-cache

Before the PR, the module file look like this.

 "variants": [
  {
    "name": "debugVariantMavenApiPublication",
    "attributes": {
      "com.android.build.api.attributes.BuildTypeAttr": "debug"
      (other attrs redacted for clarity ...)
    },
  }
]

After this PR, the module generated by the plugin should look like the 1.0.4 version module file. You can attest that by checking the diff between the 1.0.4 version and the fixed module file 1.0.5: https://www.diffchecker.com/dn2JxDwe/ (external website diffchecker.com).

It also avoid publishing debug artefact, that was not needed. See this PR and issue 274.

You can find documentation on AndroidSingleVariantLibrary or source code, if you want to deep-dive a little.

Non regression test done: see that it works in the demo project ReproProjectShowkase and test the sample app.