Since shader sources are now parsed on demand via `GPUShaderCreateInfo`,
sources are not available to be read via
`GPU_shader_get_builtin_shader_code`.
Currently this results in a crash as the code tries to read `NULL`
pointers.
`GPU_shader_get_builtin_shader_code` was created with the intention of
informing the user how a builtin shader works, thus "replacing"
detailed documentation.
Therefore this function doesn't really have a practical use in an addon.
So, instead of updating the function (which would require several
changes to the gpu module), remove it and improve the documentation.
Release Notes: https://wiki.blender.org/wiki/Reference/Release_Notes/3.2/Python_API#Breaking_Changes
Reviewed By: fclem
Differential Revision: https://developer.blender.org/D14678
3e98331a09