[libclc] Delete .gitignore (#147939)

The file is listing build artifacts to ignore, but LLVM has long had the
policy that in-tree builds are not supported, so the ignore rules
shouldn't serve their original purpose anymore.
The rules however are annoying because although they probably intended
only to ignore top-level build artifacts, they lack the leading `/` so
they match any file with the ignored name anywhere under `libclc/`.

Author: Maetveis