Remove unused `check_verbose()` method

Added in 32402827d8e738c7c4e6025b7b3bbc6d7a1a45d2 but doesn't appear to be used. aa542585aeeef589b1ce1e18edf3d4aa30a06786 removed the verbosity argument
Merge pull request #1161 from wp-cli/rm-verbose

Remove unused `check_verbose()` method