Remove `//------...` horizontal separator comments

Modern editors provide plenty of ways to visually separate functions.
Drop the explicit comments that previously served this purpose.
Use the following command to automate the change:

    $ git ls-files -z -- \
        "*.c" "*.cc" "*.cpp" "*.cxx" "*.h" "*.hh" "*.hpp" "*.hxx" |
      egrep -z -v "^Source/cmCommandArgumentLexer\." |
      egrep -z -v "^Source/cmCommandArgumentParser(\.y|\.cxx|Tokens\.h)" |
      egrep -z -v "^Source/cmDependsJavaLexer\." |
      egrep -z -v "^Source/cmDependsJavaParser(\.y|\.cxx|Tokens\.h)" |
      egrep -z -v "^Source/cmExprLexer\." |
      egrep -z -v "^Source/cmExprParser(\.y|\.cxx|Tokens\.h)" |
      egrep -z -v "^Source/cmFortranLexer\." |
      egrep -z -v "^Source/cmFortranParser(\.y|\.cxx|Tokens\.h)" |
      egrep -z -v "^Source/cmListFileLexer\." |
      egrep -z -v "^Source/cm_sha2" |
      egrep -z -v "^Source/(kwsys|CursesDialog/form)/" |
      egrep -z -v "^Utilities/(KW|cm).*/" |
      xargs -0 sed -i '/^\(\/\/---*\|\/\*---*\*\/\)$/ {d;}'

This avoids modifying third-party sources and generated sources.