Remove unnecessary modifiers from interfaces

Remove unnecessary modifiers from interfaces

From Oracle's "Defining an interface":

"All abstract, default, and static methods in an interface are
implicitly public, so you can omit the public modifier."

(Without any modifier, the interface methods are also abstract, so we
omit also the "abstract")

"In addition, an interface can contain constant declarations. All
constant values defined in an interface are implicitly public, static,
and final. Once again, you can omit these modifiers."

This makes the code more consistent. Now all interfaces under
org.eclipse.jgit follow the guidelines.

Change-Id: I4fe6deb111899ec1b4318ab5a6050f3851fa1fd3
Signed-off-by: Ivan Frade <ifrade@google.com>