Revert "Deprecate ::BlockGSStep"

This reverts commit 2967163a.

ProjectedBlockGSStep and LineGSStep inherit from it. The former is
scheduled to be deprecated soon, too, but the latter at least was
not on my radar at all. Reverting now instead of spamming everyone
with unnecessary deprecation warnings to the point of desensitization

dune/solvers/iterationsteps/blockgsstep.hh

@@ -3,8 +3,6 @@
 #ifndef DUNE_BLOCK_GAUSS_SEIDEL_STEP_HH
 #define DUNE_BLOCK_GAUSS_SEIDEL_STEP_HH
 
-#warning This header is deprecated. Please use Dune::Solvers::BlockGSStep from blockgssteps.hh instead of ::BlockGSStep. See also gssteptest.cc for usage examples.
-
 #include <dune/common/bitsetvector.hh>
 
 #include "lineariterationstep.hh"