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/ffi.rs

@@ -1201,12 +1201,6 @@ extern "C" {
         Attrs: *const &'a Attribute,
         AttrsLen: size_t,
     );
-    pub fn LLVMRustRemoveFunctionAttributes(
-        Fn: &Value,
-        index: c_uint,
-        Attrs: *const AttributeKind,
-        AttrsLen: size_t,
-    );
 
     // Operations on parameters
     pub fn LLVMIsAArgument(Val: &Value) -> Option<&Value>;