doc: don't compile the exact same snippet file twice.

It's included in the MagnumGL library because it shouldn't be compiled
if MAGNUM_TARGET_GL is disabled. So, in other words, this was breaking
the GL-less build.

doc/snippets/CMakeLists.txt

@@ -107,8 +107,7 @@ endif()
 
 if(MAGNUM_WITH_MESHTOOLS)
     add_library(snippets-MagnumMeshTools STATIC ${EXCLUDE_FROM_ALL_IF_TEST_TARGET}
-        MagnumMeshTools.cpp
-        MagnumMeshTools-gl.cpp)
+        MagnumMeshTools.cpp)
     target_link_libraries(snippets-MagnumMeshTools PRIVATE MagnumMeshTools)
     if(CORRADE_TESTSUITE_TEST_TARGET)
         add_dependencies(${CORRADE_TESTSUITE_TEST_TARGET} snippets-MagnumMeshTools)