Remove UnusedImports after x-pack feature flag removed from yaml test

[tlfeng]

Some unused imports need to be removed after the commit a40924f which removed x-pack feature flag from yaml test

* What went wrong:
Execution failed for task ':test:framework:checkstyleMain'.
> Checkstyle rule violations were found.
  Checkstyle files with violations: 2
  Checkstyle violations by severity: [error:5]

> Task :test:framework:checkstyleMain FAILED
[ant:checkstyle] [ERROR] /home/ftianli/github/search/test/framework/src/main/java/org/elasticsearch/test/rest/ESRestTestCase.java:25:8: Unused import - org.apache.http.client.methods.HttpGet. [UnusedImports]
[ant:checkstyle] [ERROR] /home/ftianli/github/search/test/framework/src/main/java/org/elasticsearch/test/rest/ESRestTestCase.java:32:8: Unused import - org.apache.logging.log4j.message.ParameterizedMessage. [UnusedImports]
[ant:checkstyle] [ERROR] /home/ftianli/github/search/test/framework/src/main/java/org/elasticsearch/test/rest/ESRestTestCase.java:105:15: Unused import - org.hamcrest.Matchers.containsString. [UnusedImports]
[ant:checkstyle] [ERROR] /home/ftianli/github/search/test/framework/src/main/java/org/elasticsearch/test/rest/ESRestTestCase.java:109:15: Unused import - org.hamcrest.Matchers.notNullValue. [UnusedImports]
[ant:checkstyle] [ERROR] /home/ftianli/github/search/test/framework/src/main/java/org/elasticsearch/test/rest/yaml/Features.java:23:8: Unused import - org.elasticsearch.test.rest.ESRestTestCase. [UnusedImports]