Remove RenderEmojiPlain from template helper (#29375)

RenderEmojiPlain(emoji.ReplaceAliases) should be called explicitly for
some contents, but not for everything.

Actually in modern days, in most cases it doesn't need such
"ReplaceAliases". So only keep it for issue/PR titles.

If anyone really needs to do ReplaceAliases for some contents, I will
propose a following fix.

templates/repo/issue/choose.tmpl

@@ -11,8 +11,8 @@
 			<div class="ui attached segment">
 				<div class="ui two column grid">
 					<div class="column left aligned">
-						<strong>{{.Name | RenderEmojiPlain}}</strong>
-						<br>{{.About | RenderEmojiPlain}}
+						<strong>{{.Name}}</strong>
+						<br>{{.About}}
 					</div>
 					<div class="column right aligned">
 						<a href="{{$.RepoLink}}/issues/new?template={{.FileName}}{{if $.milestone}}&milestone={{$.milestone}}{{end}}{{if $.project}}&project={{$.project}}{{end}}" class="ui primary button">{{ctx.Locale.Tr "repo.issues.choose.get_started"}}</a>
@@ -24,8 +24,8 @@
 			<div class="ui attached segment">
 				<div class="ui two column grid">
 					<div class="column left aligned">
-						<strong>{{.Name | RenderEmojiPlain}}</strong>
-						<br>{{.About | RenderEmojiPlain}}
+						<strong>{{.Name}}</strong>
+						<br>{{.About}}
 					</div>
 					<div class="column right aligned">
 						<a href="{{.URL}}" class="ui primary button">{{svg "octicon-link-external"}} {{ctx.Locale.Tr "repo.issues.choose.open_external_link"}}</a>