Subset comparison was introduced with the rather large commit
bf3cd41b5d, but I now think we should use
equality.
In other words, the following key binding:
bindsym Mod4+x nop Mod4+x
previously would have been triggered when pressing Mod3+Mod4+x.
Strictly speaking, this is a change of behavior, but it breaks none of our
tests, and using equality instead of subset comparison enables more use-cases.
fixes#2002
a4f6387911