tests: remove the per-tile general-product reference kernels

contraction_helpers.h (kernels::{s_t_t,t_s_t,t_t_t,t_tot_tot,tot_t_tot,
tot_tot_tot}_contract_) and its dedicated tests were scaffolding used only to
develop and cross-check einsum; the expression-layer general-product engine
has superseded them and no production code references them. Remove the header,
the generated test files, their CMake entries, the stale einsum.cpp include,
and the generate_answers.ipynb generator.

Author: evaleev

bin/generate_answers.ipynb

@@ -110,7 +110,6 @@ TiledArray/expressions/binary_expr.h
 TiledArray/expressions/blk_tsr_engine.h
 TiledArray/expressions/blk_tsr_expr.h
 TiledArray/expressions/cont_engine.h
-TiledArray/expressions/contraction_helpers.h
 TiledArray/expressions/expr.h
 TiledArray/expressions/expr_engine.h
 TiledArray/expressions/expr_trace.h

src/CMakeLists.txt

@@ -93,12 +93,6 @@ set(ta_test_src_files  ta_test.cpp
     tot_expressions.cpp
     annotation.cpp
     diagonal_array.cpp
-    contraction_helpers.cpp
-    s_t_t_contract_.cpp
-    t_t_t_contract_.cpp
-    t_s_t_contract_.cpp
-  # t_tot_tot_contract_.cpp
-  # tot_tot_tot_contract_.cpp
     einsum.cpp
     general_product.cpp
     linalg.cpp