consts: remove dead code around i1 constant values

`LLVMConstZext` recently got deleted, and it turns out (thanks to @nikic
for knowing!) that this is dead code. Tests all pass for me without this
logic, and per nikic:

> We always generate constants in "relocatable bag of bytes"
> representation, so you're never going to get a plain bool.

So this should be a safe thing to do.

r? @nikic
@rustbot label: +llvm-main

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -969,7 +969,6 @@ extern "C" {
         ConstantIndices: *const &'a Value,
         NumIndices: c_uint,
     ) -> &'a Value;
-    pub fn LLVMConstZExt<'a>(ConstantVal: &'a Value, ToType: &'a Type) -> &'a Value;
     pub fn LLVMConstPtrToInt<'a>(ConstantVal: &'a Value, ToType: &'a Type) -> &'a Value;
     pub fn LLVMConstIntToPtr<'a>(ConstantVal: &'a Value, ToType: &'a Type) -> &'a Value;
     pub fn LLVMConstBitCast<'a>(ConstantVal: &'a Value, ToType: &'a Type) -> &'a Value;