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 <bradfitz@golang.org>