Drop symbolic interface from docs

doc/index.rst

@@ -10,7 +10,6 @@ Contents:
     dof_desc
     geometry
     operators
-    symbolic
     misc
     🚀 Github <https://github.com/inducer/grudge>
     💾 Download Releases <https://pypi.org/project/grudge>

doc/symbolic.rst

-Symbolic Operator Representation
-================================
-
-Based on :mod:`pymbolic`.
-
-Basic Objects
--------------
-
-.. automodule:: grudge.symbolic.primitives
-
-Operators
----------
-
-.. automodule:: grudge.symbolic.operators