Remove git command existence check again

With 33c6dd2cc8, it's no longer necessary.

This is what I got on openbsd-current:

```
-bash-5.1$ git -v
unknown option: -v
usage: git [--version] [--help] [-C <path>] [-c <name>=<value>]
           [--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]
           [-p | --paginate | -P | --no-pager] [--no-replace-objects] [--bare]
           [--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]
           [--super-prefix=<path>] [--config-env=<name>=<envvar>]
           <command> [<args>]
-bash-5.1$ git version
git version 2.35.1
```

tool/lib/vcs.rb

@@ -397,10 +397,7 @@ class VCS
   end
 
   class GIT < self
-    register(".git") do |path, dir|
-      File.exist?(File.join(path, dir)) &&
-        (`#{COMMAND} -v` rescue false) # make sure git command exists
-    end
+    register(".git") { |path, dir| File.exist?(File.join(path, dir)) }
     COMMAND = ENV["GIT"] || 'git'
 
     def cmd_args(cmds, srcdir = nil)