uninstall: trust tab data for dependency checks

Refactor missing_dependencies to read directly from tab data
instead of falling back to formula definitions when tab data is
nil. This makes uninstall consistent with autoremove — if the tab
doesn't list a dependency, neither command blocks on it.

Previously, autoremove would suggest removing a package (tab says
it's unused), but uninstall would refuse (formula definition says
it's needed). This happened when homebrew-core added a new
dependency to a formula after the bottle was installed, since the
tab is written at install time and never updated.

The autoremove cross-check against formula definitions is removed
as it's no longer needed — both commands now trust the same data
source.