[Support] Remove the migration helpers to llvm::endianness

I've migrated all known uses of llvm::support::endianness to
llvm::endianness.  This patch removes the migration helpers.

Author: kazutakahirata

llvm/include/llvm/Support/Endian.h

@@ -25,13 +25,6 @@
 namespace llvm {
 namespace support {
 
-// TODO: Remove the following once we are done migrating to llvm::endianness,
-// llvm::endianness::big, etc.
-using endianness = llvm::endianness;
-constexpr llvm::endianness big = llvm::endianness::big;
-constexpr llvm::endianness little = llvm::endianness::little;
-constexpr llvm::endianness native = llvm::endianness::native;
-
 // These are named values for common alignments.
 enum {aligned = 0, unaligned = 1};
 
@@ -47,10 +40,6 @@ struct PickAlignment {
 
 namespace endian {
 
-LLVM_DEPRECATED("Use llvm::endianness::native instead",
-                "llvm::endianness::native")
-constexpr endianness system_endianness() { return llvm::endianness::native; }
-
 template <typename value_type>
 [[nodiscard]] inline value_type byte_swap(value_type value, endianness endian) {
   if (endian != llvm::endianness::native)