llvmjit: Fix building against LLVM 11 by removing unnecessary include.

LLVM has removed this header, in the branch that will become llvm
11. But as it turns out we didn't actually need it, so just remove it.

Author: Jesse Zhang <sbjesse@gmail.com>
Discussion: https://postgr.es/m/CAGf+fX7bvtP0YXMu7pOsu_NwhxW6dArTkxb=jt7M2-UJkyJ_3g@mail.gmail.com
Backpatch: 11, where JIT support using llvm was introduced.

src/backend/jit/llvm/llvmjit_inline.cpp

@@ -56,7 +56,6 @@ extern "C"
 #include <llvm/Support/Error.h>
 #endif
 #include <llvm/IR/Attributes.h>
-#include <llvm/IR/CallSite.h>
 #include <llvm/IR/DebugInfo.h>
 #include <llvm/IR/IntrinsicInst.h>
 #include <llvm/IR/IRBuilder.h>