Remove '--mixed' from the 'gir' alias

Fix a bug introduced in d4a5bed.

Though '--mixed' is the default for 'git reset', using it with paths
returns the following warning.

warning: --mixed with paths is deprecated; use 'git reset -- <paths>'
instead.

modules/git/alias.zsh

@@ -53,7 +53,7 @@ alias giA='git add --patch'
 alias giu='git add --update'
 alias gid='git diff --no-ext-diff --cached'
 alias giD='git diff --no-ext-diff --cached --word-diff'
-alias gir='git reset --mixed'
+alias gir='git reset'
 alias giR='git reset --keep'
 alias gix='git rm -r --cached'
 alias giX='git rm -rf --cached'