test/crimson: drop copy assignment operator

test/crimson: drop copy assignment operator

because of the const reference member variable of `parent`:

const delta_overlay_t &parent;

we are not allowed to mutate it. so the explicitly defaulted copy assignment
operator is implicitly deleted

this change silences the warnings from Clang like:

 ../src/test/crimson/seastore/test_transaction_manager.cc:159:12: warning: explicitly defaulted copy assignment operator is implicitly deleted [-Wdefaulted-function-deleted]
         iterator &operator=(const iterator &) = default;
                   ^
 ../src/test/crimson/seastore/test_transaction_manager.cc:98:25: note: copy assignment operator of 'iterator' is implicitly deleted because field 'parent' is of reference type 'const transaction_manager_$
         const delta_overlay_t &parent;
                                ^

Signed-off-by: Kefu Chai <kchai@redhat.com>