Remove LLVM attribute removal

This was necessary before, because `declare_raw_fn` would always apply
the default optimization attributes to every declared function,
and then `attributes::from_fn_attrs` would have to remove the default
attributes in the case of, e.g. `#[optimize(speed)]` in a `-Os` build.

However, every relevant callsite of `declare_raw_fn` (i.e. where we
actually generate code for the function, and not e.g. a call to an
intrinsic, where optimization attributes don't [?] matter)
calls `from_fn_attrs`, so we can simply remove the attribute setting
from `declare_raw_fn`, and rely on `from_fn_attrs` to apply the correct
attributes all at once.

compiler/rustc_codegen_llvm/src/llvm/mod.rs

@@ -37,12 +37,6 @@ pub fn AddFunctionAttributes<'ll>(llfn: &'ll Value, idx: AttributePlace, attrs:
     }
 }
 
-pub fn RemoveFunctionAttributes(llfn: &Value, idx: AttributePlace, attrs: &[AttributeKind]) {
-    unsafe {
-        LLVMRustRemoveFunctionAttributes(llfn, idx.as_uint(), attrs.as_ptr(), attrs.len());
-    }
-}
-
 pub fn AddCallSiteAttributes<'ll>(
     callsite: &'ll Value,
     idx: AttributePlace,