patch 9.1.1550: defaults: 'showcmd' is not enabled in non-compatible mode on Unix

patch 9.1.1550: defaults: 'showcmd' is not enabled in non-compatible mode on Unix

Problem:  defaults: 'showcmd' is not enabled in non-compatible mode on
          Unix
Solution: Always enable 'showcmd' in non-compatible mode, drop it from
          defaults.vim.

'showcmd' was already always enabled in Vim compatible mode except for
UNIX environments. So let's just enable it always, there is no good
reason why UNIX platforms should be handled differently than other
platforms, especially since `defaults.vim` did enable this option
anyhow.

closes: #17739

Signed-off-by: Christian Brabandt <cb@256bit.org>