Stop explicitly creating a sourcesElement Gradle variant

This variant has been superceded by an equivalent
`releaseVariantReleaseRuntimePublication` variant which (I think) has
been created since d5f9cf4.

Before this change the `sourcesElement` variant was being created with
no `files` config, which resulted in build failures when generating
javadoc in AndroidX:

```
However we cannot choose between the following variants of androidx.media3:media3-cast:1.2.0-alpha01:
  - releaseVariantReleaseSourcePublication
  - sourcesElement
```

We can resolve this confusion by deleting the empty & unneeded
`sourcesElement` variant.

#minor-release

PiperOrigin-RevId: 558783471
(cherry picked from commit 849238a)

publish.gradle

@@ -29,23 +29,6 @@ afterEvaluate {
                     groupId = 'androidx.media3'
                     artifactId = findProperty('releaseArtifactId') ?: ''
                     version = findProperty('releaseVersion') ?: ''
-                    configurations.create("sourcesElement") { variant ->
-                        variant.visible = false
-                        variant.canBeResolved = false
-                        variant.attributes.attribute(
-                            Usage.USAGE_ATTRIBUTE,
-                            project.objects.named(Usage, Usage.JAVA_RUNTIME))
-                        variant.attributes.attribute(
-                            Category.CATEGORY_ATTRIBUTE,
-                            project.objects.named(Category, Category.DOCUMENTATION))
-                        variant.attributes.attribute(
-                            Bundling.BUNDLING_ATTRIBUTE,
-                            project.objects.named(Bundling, Bundling.EXTERNAL))
-                        variant.attributes.attribute(
-                            DocsType.DOCS_TYPE_ATTRIBUTE,
-                            project.objects.named(DocsType, DocsType.SOURCES))
-                        components.release.addVariantsFromConfiguration(variant) {}
-                    }
                     pom {
                         name =
                             findProperty('releaseName')