terminal: fix crash in header reporting when absolute testpaths is used
Regressed in 6.1.0 in 62e249a1f934d1073c9a0167077e133c5e0f6270.
The `x` is an `str` but is expected to be a `pathlib.Path`. Not caught
by mypy because `config.getini()` returns `Any`.
Fix by just removing the `bestrelpath` call:
- testpaths are always relative to the rootdir, it thus would be very
unusual to specify an absolute path there.
- The code was wrong even before the regression: `py.path.local`'s
`bestrelpath` function expects a `py.path.local`, not an `str`. But it
had some weird `try ... except AttributeError` fallback which just
returns the argument, i.e. it was a no-op. So there is no behavior
change.
- It seems reasonable to me to just print the full path if that's what
the ini specifies.
(cherry picked from commit 61f80a783a38441040c5999a4033af8004ee4c6b)