Remove 'abstract' modifier from methods in interfaces and annotations.
Remove 'abstract' modifier from methods in interfaces and annotations.

Summary:
All methods in interfaces and annotations are abstract by definition,
so it's not useful to show.

Related to https://github.com/material-motion/material-motion-apidiff/issues/9

Sample output:

## StringRemixMethod

*new* annotation: `@StringRemixMethod`

*new* method: `String defaultValue()`

*new* method: `String key()`

*new* method: `int layoutId()`

*new* method: `String title()`

Reviewers: O2 Material Motion, featherless

Reviewed By: O2 Material Motion, featherless

Subscribers: featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D1515

material-motion/apidiff,material-motion/apidiff,material-motion/apidiff