Remove bogus virSecurityManagerSetProcessFDLabel method

Remove bogus virSecurityManagerSetProcessFDLabel method

The virSecurityManagerSetProcessFDLabel method was introduced
after a mis-understanding from a conversation about SELinux
socket labelling. The virSecurityManagerSetSocketLabel method
should have been used for all such scenarios.

* src/security/security_apparmor.c, src/security/security_apparmor.c,
  src/security/security_driver.h, src/security/security_manager.c,
  src/security/security_manager.h, src/security/security_selinux.c,
  src/security/security_stack.c: Remove SetProcessFDLabel driver