encoding/base32: remove redundant conditional

encoding/base32: remove redundant conditional

Immediately following the conditional block removed here is a loop
which checks exactly what the conditional already checked, so the
entire conditional is redundant.

Change-Id: I892fd9f2364d87e2c1cacb0407531daec6643183
Reviewed-on: https://go-review.googlesource.com/114000
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>