rockchip: Remove obsolete references to pyelftools

make_fit_atf.py no longer requires pyelftools, and nothing else in the
rockchip build requires it either, so remove references to installing it
from the documentation.

Signed-off-by: Chris Webb <chris@arachsys.com>
Reviewed-by: Kever Yang <Kever.yang@rock-chips.com>

board/rockchip/evb_rk3399/README

@@ -35,12 +35,6 @@ Get the Source and prebuild binary
   > git clone https://github.com/rockchip-linux/rkbin.git
   > git clone https://github.com/rockchip-linux/rkdeveloptool.git
 
-Get some prerequisites
-======================
-
-You need the Python elftools.elf.elffile library for make_fit_atf.py to work:
-
-  > sudo apt-get install python-pyelftools
 
 Compile ATF
 ===========

doc/README.rockchip

@@ -123,10 +123,6 @@ For example:
 
    Option 2: Package the image with SPL:
 
-   - We need the Python elftools.elf.elffile library for make_fit_atf.py to work
-
-     => sudo apt-get install python-pyelftools
-
    - Export cross compiler path for aarch64
 
    - Compile ATF