Get rid of some unused friendship declarations.

Get rid of some unused friendship declarations.

Clang warns about the second one with the following:

/home/drwells/Documents/Code/CPP/dealii-dev-clang/include/deal.II/lac/sparse_matrix.h:1645:93: warning:
      dependent nested name specifier 'SparseMatrixIterators::Accessor<number2, false>::' for friend
      class declaration is not supported; turning off access control for 'SparseMatrix'
      [-Wunsupported-friend]
  template <typename number2> friend class SparseMatrixIterators::Accessor<number2, false>::Reference;

Additionally, MSVC runs into errors with this friendship
declaration (and can compile the library without it), so we can get rid
of it.