Buck: Adjust to upstream change: "default java targeted to be Java 7"

60efceb upstream change flips the default version of java targeted to be
Java 7 rather than the EoL'd Java 6.  Now we don't need to overwrite
Buck's own python methods to enforce Java 7 to be the default Java version.

Moreover, projects that want to switch to newer Java version (e. g. Java 8)
can do it now in .buckconfig file:

  [java]
    source_level = 8
    target_level = 8

[1] https://github.com/facebook/buck/commit/60efceb5239134d709be4087916e95e07542519a

Change-Id: I3e22b798e18b013a5b43922cb0fdf601f76921a7