Drop support for sentencepiece

[gonsolo]

Hi!

Drop support for sentencepiece or at least make it optional as it

1. doesn't work with GCC 15
2. doesn't work with CMake 4
3. is unresponsive
4. last released one and a half years ago.

It fails to install from PYPI on modern machines, makes your package fail to install, and a lot of other packages that depend on gguf or sentencepiece.

See a lot of issues that are not addressed here:
https://github.com/google/sentencepiece/issues