Remove sbt-ghpages because it is pulled in by sbt-microsites (#1852)

sbt-ghpages is a dependency of sbt-microsites and therefore does
not need to be listed explicitly as plugin. We have the same situation
with tut whose keys we are using in our `build.sbt` but do not explicitly
list in `plugins.sbt`.

project/plugins.sbt

@@ -3,7 +3,6 @@ addSbtPlugin("com.eed3si9n"        % "sbt-unidoc"            % "0.3.3")
 addSbtPlugin("com.github.gseitz"   % "sbt-release"           % "1.0.5")
 addSbtPlugin("com.jsuereth"        % "sbt-pgp"               % "1.0.0")
 addSbtPlugin("com.typesafe"        % "sbt-mima-plugin"       % "0.1.17")
-addSbtPlugin("com.typesafe.sbt"    % "sbt-ghpages"           % "0.5.3")
 addSbtPlugin("pl.project13.scala"  % "sbt-jmh"               % "0.2.27")
 addSbtPlugin("org.scalastyle"      % "scalastyle-sbt-plugin" % "0.8.0")
 addSbtPlugin("org.scoverage"       % "sbt-scoverage"         % "1.5.0")