Remove powermock

Powermock was introduced in I5a38a5fd3, but is not used in any of the
core tests or core plugins' tests. It is however used by some non-core
plugins (for example its-base, its-phabricator, owners) and those will
need to be migrated away from powermock, as was already done for the
slack-integration plugin in change I7595a8359, or explicitly add it to
their external dependencies.

Also remove javassist which was added as a transitive dependency at the
same time powermock was added.

Feature: Issue 5057
Change-Id: Iafcc40d2fafa58b9262b318d0207cff99239bd15

java/com/google/gerrit/testing/BUILD

@@ -9,12 +9,8 @@ java_library(
     ),
     visibility = ["//visibility:public"],
     exports = [
+        "//lib:junit",
         "//lib/easymock",
-        "//lib/powermock:powermock-api-easymock",
-        "//lib/powermock:powermock-api-support",
-        "//lib/powermock:powermock-core",
-        "//lib/powermock:powermock-module-junit4",
-        "//lib/powermock:powermock-module-junit4-common",
     ],
     deps = [
         "//java/com/google/gerrit/acceptance/testsuite/project",