Del redundant-static-def in executorch/schema/extended_header.cpp +2 (pytorch#13875)

Summary:

LLVM has a warning `-Wdeprecated-redundant-constexpr-static-def` which raises the warning:

> warning: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated

Since we are now on C++20, we can remove the out-of-line definition of constexpr static data members. This diff does so.

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Differential Revision: D81359733

Author: r-barnes

schema/extended_header.cpp

@@ -91,8 +91,5 @@ uint64_t GetUInt64LE(const uint8_t* data) {
   };
 }
 
-// Define storage for the static.
-constexpr char ExtendedHeader::kMagic[kMagicSize];
-
 } // namespace runtime
 } // namespace executorch