refactor(downGithub.ps1): 移除OnlyBackup参数并调整WithDate格式

移除不再使用的OnlyBackup参数，并调整WithDate参数的格式以保持代码整洁

Author: mudssky

downGithub.ps1

@@ -31,10 +31,10 @@ param (
 	[string]
 	$Path = '.',
 	[switch]
-	$WithDate,
+	$WithDate
 	# 纯备份场景不需要工作区
-	[switch]
-	$OnlyBackup
+	# [switch]
+	# $OnlyBackup
 )
 
 # $repos = Invoke-RestMethod -Uri "https://api.github.com/users/$UserName/repos" -Headers @{ "User-Agent" = "Mozilla/5.0" }