Xserver/os/oscolor.c: Follow X.org in dropping the rgb.txt file entirely. Content of that file is now hard-coded.

 Adopting an exact copy of os/oscolor.c from X.org leads to this compiler warning:

 ```
 oscolor.c:844:1: warning: string length ‘7614’ is greater than the length ‘4095’ ISO C99 compilers are required to support [-Woverlength-strings]
 };
 ^
 ```

 This will be address later in coordination with X.org.

 Note: there is a tiny change in oscolor.c as we ship it. The original X.org file includes <X11/keysym.h> which seems
 unneeded. We drop that one line from oscolor.c.

 Fixes ArcticaProject/nx-libs#259.