Remove unnecessary find_package(OpenGL)

Before this change, CMakeLists.txt would run find_package(OpenGL), but
it would never use any of the targets or variables that calling
find_package(OpenGL) produces [1]. As a result, the call to
find_package(OpenGL) didn’t really do anything.

[1]: <https://cmake.org/cmake/help/latest/module/FindOpenGL.html>

CMakeLists.txt

@@ -89,7 +89,6 @@ if(UNIX)
 
   find_package(SDL2 REQUIRED)
   find_package(Curses REQUIRED)
-  find_package(OpenGL REQUIRED)
   # Some versions of the SDL2 find_package set SDL2_INCLUDE_DIR and some set a plural SDL2_INCLUDE_DIRS. Check both.
   message("SDL2 Include Dir is ${SDL2_INCLUDE_DIR} ${SDL2_INCLUDE_DIRS}")
 endif()