Remove SDK/common/vector.cpp & vector.h from all projects.

Move utility functions into the modules where they "belong to":
- vec3_rotate_quat() & quat_build_r() are utility functions that belong
  in the MD5 module.
- vec3_to_recast() & recast_to_vec3() are utility functions that belong
  in the NAVIGATION module.
Update README.md file.