test: at: Remove dead code

Remove two expressions from at_cellularcontexttest that do nothing. This
fixes the following two warnings.

    connectivity/cellular/tests/UNITTESTS/framework/AT/at_cellularcontext/at_cellularcontexttest.cpp:61:67: warning: expression result unused [-Wunused-value]            ATHandler_stub::int_valid_count_table[kRead_int_table_size];                                                                                                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~^
    connectivity/cellular/tests/UNITTESTS/framework/AT/at_cellularcontext/at_cellularcontexttest.cpp:64:66: warning: expression result unused [-Wunused-value]            ATHandler_stub::read_string_table[kRead_string_table_size];

Author: Patater

connectivity/cellular/tests/UNITTESTS/framework/AT/at_cellularcontext/at_cellularcontexttest.cpp

@@ -58,10 +58,8 @@ class TestAT_CellularContext : public testing::Test {
         ATHandler_stub::call_immediately = false;
         ATHandler_stub::resp_info_true_counter = false;
         ATHandler_stub::info_elem_true_counter = false;
-        ATHandler_stub::int_valid_count_table[kRead_int_table_size];
         ATHandler_stub::int_count = kRead_int_table_size;
         ATHandler_stub::read_string_index = kRead_string_table_size;
-        ATHandler_stub::read_string_table[kRead_string_table_size];
         ATHandler_stub::resp_stop_success_count = kResp_stop_count_default;
         CellularDevice_stub::connect_counter = 2;