[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/`.

libclc/.gitignore

@@ -1,15 +0,0 @@
-Makefile
-amdgcn--
-amdgcn--amdhsa
-amdgcn-mesa-mesa3d
-build/*.pyc
-built_libs/
-generic--
-generic/lib/convert.cl
-libclc.pc
-nvptx--nvidiacl
-nvptx64--nvidiacl
-r600--
-utils/prepare-builtins
-utils/prepare-builtins.o
-utils/prepare-builtins.o.d