Remove badges from documentation (#162)

The badges are given a prominent position at the top of the user
documentation but are of very little value to the user.  Those who
want to know the current release can see it very clearly on the plugins
site https://plugins.jenkins.io/plot/releases/ and on the GitHub page
https://github.com/jenkinsci/plot-plugin/releases .

The coverage information from the codecov badge is available from the
coverage report on ci.jenkins.io and from the coverage check report
on GitHub.  One more location for coverage reporting is not valuable
enough to occupy the top line of the user documentation.

The installation count is provided on the plugins.jenkins.io page and
includes a 12 month history graph and easy access to detailed deployment
statistics by plugin version and Jenkins core version.

README.md

@@ -1,12 +1,5 @@
 # Plot plugin
 
-[![codecov](https://codecov.io/gh/jenkinsci/plot-plugin/branch/master/graph/badge.svg)](https://codecov.io/gh/jenkinsci/plot-plugin)
-[![Jenkins Plugins](https://img.shields.io/jenkins/plugin/v/plot)](https://plugins.jenkins.io/plot)
-[![GitHub Release](https://img.shields.io/badge/dynamic/json?color=blue&label=changelog&query=$.tag_name&url=https://api.github.com/repos/jenkinsci/plot-plugin/releases/latest)](https://github.com/jenkinsci/plot-plugin/releases/latest)
-[![Jenkins Plugin installs](https://img.shields.io/jenkins/plugin/i/plot?color=blue)](https://plugins.jenkins.io/plot/)
-
-## Description
-
 This plugin provides generic plotting (or graphing) capabilities in Jenkins.
 
 This plugin will **plot** one or more **single values variations across builds** in one or more plots.