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>

runtime/doc/version9.txt

@@ -1,4 +1,4 @@
-*version9.txt*  For Vim version 9.1.  Last change: 2025 Jul 14
+*version9.txt*  For Vim version 9.1.  Last change: 2025 Jul 15
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -41648,6 +41648,9 @@ Default values: ~
 - the default value of the 'keyprotocol' option has been updated and support
   for the ghostty terminal emulator (using kitty protocol) has been added
 - Improved visual highlighting |hl-Visual|
+- the default value for 'showcmd' is always enabled when using non-compatible
+  mode (previously, it was off on UNIX systems) and consequently removed from
+  |defaults.vim|
 
 Completion: ~
 - allow to complete directories from 'cdpath' for |:cd| and similar commands,