Update to glfw/glfw@3b9a14f.

- Remove `glfw_joystick.c`, `glfw_time.c` files since their equivalents were removed in GLFW C library.
- Remove unnecessary CMake, example, documentation and test files of GLFW C library.

According to [`glfw/COPYING.txt`](https://github.com/glfw/glfw/blob/master/COPYING.txt),

> \2. Altered source versions must be plainly marked as such, and must not
>      be misrepresented as being the original software.

Since I've modified the source distribution (to remove unneeded files and avoid future diffs showing changes in irrelevant files), should we add a note to clarify that?

```Markdown
GLFW C Library Source
=====================

This is a limited source copy of the GLFW C library. It's embedded inside the glfw3 Go package to make it easy to `go get`.

The original GLFW C library source is available at http://www.glfw.org/.
```