[lldb] Omit --show-globals in `help target var` (#85855)

This option doesn't exist. It is currently displayed by `help target
var` due to a bug introduced by 41ae8e74 in 2018.

Some code for `target var` and `frame var` is shared, and some hard-code
constants are used in order to filter out options that belong only to
`frame var`. However, the aforementioned commit failed to update these
constants properly. This patch addresses the issue by having a _single_
place where the filtering of options needs to be done.