[ObjectYAML] Remove redundant declarations (NFC)

In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/lib/ObjectYAML/ELFYAML.cpp

@@ -37,8 +37,6 @@ unsigned Object::getMachine() const {
     return *Header.Machine;
   return llvm::ELF::EM_NONE;
 }
-
-constexpr StringRef SectionHeaderTable::TypeStr;
 } // namespace ELFYAML
 
 namespace yaml {