text/template: delete obsolete nil check

This was added during testing but is unnecessary.
Thanks to gravis on GitHub for catching it.

See #10574.

Change-Id: I4a8f76d237e67f5a0ea189a0f3cadddbf426778a
Reviewed-on: https://go-review.googlesource.com/9841
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: robpike

src/text/template/parse/lex.go

@@ -177,9 +177,6 @@ func (l *lexer) nextItem() item {
 // drain drains the output so the lexing goroutine will exit.
 // Called by the parser, not in the lexing goroutine.
 func (l *lexer) drain() {
-	if l == nil {
-		return
-	}
 	for range l.items {
 	}
 }