Fix Mdm::Module::Detail#stance bug

[#49858419]
[SEERM #7958]

metasploit_data_models 0.14.3 relaxes the validation on
Mdm::Module::Detail#stance so it only needs to be in
Mdm::Module::Detail::STANCES if Mdm::Module::Detail#mtype is 'auxiliary'
or 'exploit' as framework only supplies a stance for those types when
using Mdm::Module::Detail.