[spec] Remove lingering one-memory validation rule (WebAssembly#1848)

Author: bvisness

document/core/valid/modules.rst

@@ -841,8 +841,6 @@ The :ref:`external types <syntax-externtype>` classifying a module may contain f
 
 * Let :math:`\X{et}^\ast` be the concatenation of :ref:`external types <syntax-externtype>` :math:`\X{et}_i` of the exports, in index order.
 
-* The length of :math:`C.\CMEMS` must not be larger than :math:`1`.
-
 * All export names :math:`\export_i.\ENAME` must be different.
 
 * Then the module is valid with :ref:`external types <syntax-externtype>` :math:`\X{it}^\ast \to \X{et}^\ast`.