gc: `-c` works without scope specifier (-w,a,T etc)

[skshetry]

As the title says, dvc gc -c works without -w/-T/-a specifiers (i.e. by default, it assumes -w).
It'd be good to only get in action when those specifiers are mentioned for dvc gc -c.

Refs:

Discord discussion
#3363 made it mandatory to require use of flags.