Use externs/olx.js instead of objectliterals.jsdoc

The single `externs/olx.js` file describes all of the "options" objects we accept in our constructors.  The `@typedef` annotations are used by the compiler for type checking.  The `@type` annotations include documentation for individual options and serve as externs when compiling a profile of the library.  When compiling an application together with the library, the `externs/olx.js` file is included as one of the sources to provide the `@typedef`s without generating externs.

If we want to maintain multiple `src/*.externs.js` files instead of one large `externs/olx.js` file, we can.  But while we are still using Plovr, it makes for easier build configurations to have one file.

This removes the build tasks that generated the `build/src/external/src/externs/types.js` and `build/src/internal/src/types.js` files as those are both replaced by the single `externs/olx.js` file.