patch 9.1.1831: stray vseps in right-most 'winfixwidth' window

patch 9.1.1831: stray vseps in right-most 'winfixwidth' window

Problem:  vertical separator of 'winfixwidth' windows may remain if they
          become right-most windows from closing windows to the right.
Solution: Don't implicitly rely on frame_new_width to fix vseps, as the
          call may be skipped for 'winfixwidth' windows to preserve
          their width; do it explicitly in winframe_remove (Sean Dewar).

Note that I prefer win_new_width here over setting w_width directly, which
would've previously been done by win_split_ins after frame_add_vsep, as this
wasn't true for winframe_remove.

Though the equivalent issue of bottom 'winfixheight' windows leaving stray
statuslines with &ls=0 doesn't seem to exist, test it anyway.

closes: #18481

Signed-off-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>