explore: remove 4 line config options (#10562)

This PR removes the `line_head_top`, `line_head_bottom`, `line_shift`,
and `line_index` configuration options from `explore`. These were
previously used to control whether the horizontal+vertical lines in this
`ls | explore -i` screenshot would be displayed:


![image](https://github.com/nushell/nushell/assets/26268125/b705e8a0-935c-40ff-be4a-f119dbae3080)

Now, all lines are displayed (same as the previous default config
values) and this is no longer configurable.

## Context

I'm continuing to chip away at `explore` when I have time. I have a
long-term goal to make `explore` simpler for users+developers. For now
I'm mostly making small incremental changes where I find underused
functionality+configuration and remove it; hopefully eventually this
will make it easier to make larger changes.

I found these specific config options a little hard to understand when
reading `explore` code, and when reading `config.nu` as a user their
behaviour+naming is not obvious. I also think that in the long term,
`explore` styling should inherit most styling from `table` instead of
having its own styling system.

crates/nu-utils/src/sample_config/default_config.nu

@@ -192,10 +192,6 @@ $env.config = {
             selected_cell: {bg: light_blue},
             selected_row: {},
             selected_column: {},
-            line_head_top: true,
-            line_head_bottom: true,
-            line_shift: true,
-            line_index: true,
         },
     }