Compile warnings in deep_mind.cpp, abi_serializer.cpp, producer_plugin.cpp, auth_tests.cpp

[linh2931]

Minor warnings:

libraries/chain/deep_mind.cpp:46:4: warning: control reaches end of non-void function [-Wreturn-type]

libraries/chain/abi_serializer.cpp:80:42: warning: ‘void eosio::chain::abi_serializer::set_abi(const eosio::chain::abi_def&, const fc::microseconds&)’ is deprecated: use the overload with yield_function_t[=create_yield_function(max_serialization_time)] [-Wdeprecated-declarations]

plugins/producer_plugin/producer_plugin.cpp:1959:43: warning: comparison of integer expressions of different signedness: ‘uint64_t’ {aka ‘long unsigned int’} and ‘int64_t’ {aka  ‘long int’} [-Wsign-compare]
 1959 |                if( prev_billed_plus100_us < max_trx_time.count() ) max_trx_time = fc::microseconds( prev_billed_plus100_us );

unittests/auth_tests.cpp:235:18: warning: variable ‘new_owner_priv_key’ set but not used [-Wunused-but-set-variable]