Delete duplicate version numbers of Maven plugins

The Maven plugin version numbers are defined in the pluginManagement
of the spring-boot-dependencies pom.xml. Consequently, they can be
deleted in the spring-boot-starter-parent pom.xml to avoid version
conflicts and duplicated version management.
Merge pull request #1396 from matsev/cleaning-up-maven-plugin-dependencies

* cleaning-up-maven-plugin-dependencies:
  Delete duplicate version numbers of Maven plugins