build.gradle: remove wrapper task
build.gradle: remove wrapper task

Since Gradle 4.8, overriding built-in tasks, such as the `wrapper` task,
has been deprecated[1] and since Gradle 5.0 would actually produce an
error[2]:

    Cannot add task 'wrapper' as a task with that name already exists

Specifying the gradle version used for executing the build via creating
a "wrapper" task is not required since Gradle will use the version
specified in the `gradle/wrapper/gradle-wrapper.properties` file[3], which
we already have.

As we are planning to upgrade to Gradle 5.2.1, let's remove the wrapper
task.

[1] https://docs.gradle.org/4.8/release-notes.html#overwriting-gradle's-built-in-tasks
[2] https://docs.gradle.org/5.0/userguide/upgrading_version_4.html#potential_breaking_changes

    "Overriding built-in tasks deprecated in 4.8 now produces an error.
     Attempting to replace a built-in task will produce an error similar to
     the following:
         Cannot add task 'wrapper' as a task with that name already exists."

[3] https://docs.gradle.org/5.2.1/userguide/gradle_wrapper.html#sec:adding_wrapper

    "The generated Wrapper properties file,
     gradle/wrapper/gradle-wrapper.properties, stores the information
     about the Gradle distribution.
          The server hosting the Gradle distribution.
          The type of Gradle distribution. ...
          The Gradle version used for executing the build. ... "

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