C++: Remove @precision from AV Rule 78

This rule, named "No virtual destructor", was supposed to be superseded
by `cpp/virtual-destructor` in 0c796de, but that commit didn't
actually disable this rule, so both rules are now active in the LGTM
suite.

This commit disables the rule by removing `@precision`. We're still
discussing the best way to disable rules that are precise and valid but
not universally applicable. For now, removing `@precision` is consistent
with how we're keeping most other JSF queries from appearing on LGTM.

Author: jbj

cpp/ql/src/jsf/4.10 Classes/AV Rule 78.ql

@@ -3,7 +3,6 @@
  * @description All base classes with a virtual function should define a virtual destructor. If an application attempts to delete a derived class object through a base class pointer, the result is undefined if the base class destructor is non-virtual.
  * @kind problem
  * @problem.severity warning
- * @precision high
  * @id cpp/jsf/av-rule-78
  * @tags reliability
  *       readability