gdb: Remove deprecated assertion in setting::get

The commit 702991711a (gdb: Have setter
and getter callbacks for settings) makes it possible for a setting not
to be backed by a memory buffer but use callback functions instead to
retrieve or set the setting's value.

An assertion was not properly updated to take into account that the
m_var member (which points to a memory buffer, if used) might be nullptr
if the setting uses callback functions.  If the setting is backed by a
memory buffer, the m_var has to be non nullptr, which is already checked
before the pointer is dereferenced.

This commit removes this assertion as it is not valid anymore.

gdb/command.h

@@ -288,7 +288,6 @@ struct setting
   const T &get () const
   {
     gdb_assert (var_type_uses<T> (m_var_type));
-    gdb_assert (m_var != nullptr);
 
     if (m_var == nullptr)
       {