STYLE: remove old (__cplusplus >= 201703L) checks

applications/test/Hashing2/Make/files

applications/test/Hashing2/Test-Hashing2.cxx

applications/test/ICharStream1/Make/files

applications/test/ICharStream1/Test-ICharStream1.cxx

applications/test/ITstream/Test-ITstream.cxx

applications/test/ListRead1/Test-ListRead1.cxx

applications/test/OCharStream1/Make/files

applications/test/OCharStream1/Test-OCharStream1.cxx

applications/test/SpanStream1/Make/files

applications/test/SpanStream1/Test-SpanStream1.cxx

applications/test/dictionaryTokens/Make/files

applications/test/dictionaryTokens/Test-dictionaryTokens.cxx

applications/test/dictionaryTokens/dictionaryTokens.cxx

applications/test/string_view1/Make/files

applications/test/string_view1/Test-string_view1.cxx

applications/test/token/Test-token.cxx

applications/test/tokenize/Make/files

applications/test/tokenize/Test-tokenize.cxx

src/OpenFOAM/db/IOstreams/IOstreams/Ostream.H

src/OpenFOAM/db/IOstreams/Sstreams/OSstream.C