These function templates are explicitly specialized for char and wchar_t
streambufs, so the explicit instantiations do nothing. Remove them, to
avoid confusion.

libstdc++-v3/ChangeLog:

	* include/bits/streambuf.tcc (__copy_streambufs_eof): Remove
	explicit instantiation declarations.
	* src/c++11/streambuf-inst.cc (__copy_streambufs_eof): Remove
	explicit instantiation definitions.

Tested powerpc64le-linux. Committed to trunk.