`CD_HAIRLENGTH` is not really a custom data type, it was just used to
specify the hair particle "length" attribute to pass it to EEVEE and
material preview. Because of the "typemap" array in `CustomData`,
it's better not to have this unnecessary type. Instead, use the same
mechanism used to request the active color attribute.
Pull Request: https://projects.blender.org/blender/blender/pulls/109449
a1cc621e1e