This commit removes the precompiled headers that I added in Commit
3bcfeb700 ("cmake : add PCH and unity build to improve build times
(#28091)").
The motivation for this is that this looked good when developing this
but has caused multiple issues that I had taken into consideration and
we have decided to remove it and only keep the unity builds from the
above commit.
Refs: https://github.com/ggml-org/llama.cpp/pull/28882#issuecomment-5662272126