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.

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

Change-Id: I8735dc0bec74c55c88ddcd07fe035536e7f0ceff
Merge "Buck: Adjust to upstream change: "default java targeted to be Java 7""