Drop symbolic execution in grudge

examples/old_symbolics/dagrt-fusion.py

examples/old_symbolics/symbolic_wave_op.py

grudge/__init__.py

grudge/discretization.py

grudge/execution.py

grudge/function_registry.py

grudge/symbolic/__init__.py

grudge/symbolic/compiler.py

grudge/symbolic/dofdesc_inference.py

grudge/symbolic/mappers/__init__.py

grudge/symbolic/operators.py

grudge/symbolic/primitives.py

grudge/symbolic/tools.py

grudge/trace_pair.py

test/test_grudge.py

test/test_grudge_sym_old.py